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140 lines
4.7 KiB
140 lines
4.7 KiB
import torch
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from torch.fx import Graph, Node
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from torch.utils._pytree import tree_map
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def normalize_tuple(x):
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if not isinstance(x, tuple):
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return (x,)
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return x
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def is_autogradable(x):
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return isinstance(x, torch.Tensor) and x.is_floating_point()
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def meta_trace(module: torch.nn.Module, fake_device=None, *args, **kwargs) -> Graph:
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"""Trace forward and backward graph with MetaTensor
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Args:
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module (torch.nn.Module): The target module for tracing.
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Returns:
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graph (torch.fx.Graph): The computation graph.
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Usage:
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>>> import torchvision.models as tm
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>>> model = tm.alexnet()
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>>> graph = meta_trace(model, torch.rand(1000, 3, 224, 224))
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>>> graph.print_tabular()
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"""
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graph = Graph()
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namespace = graph._graph_namespace
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class MetaProxy(torch.Tensor):
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"""
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A wrapping tensor that hacks `torch.autograd` without patching more `torch.ops.aten` ops.
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"""
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_tensor: torch.Tensor
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_node: Node
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__slots__ = ["_tensor", "_node"]
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@staticmethod
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def __new__(cls, tensor, fake_device=None, placeholder=False, name=None):
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r = torch.Tensor._make_wrapper_subclass(
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cls,
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tensor.size(),
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strides=tensor.stride(),
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storage_offset=tensor.storage_offset(),
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dtype=tensor.dtype,
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layout=tensor.layout,
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device=fake_device if fake_device is not None else tensor.device,
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requires_grad=tensor.requires_grad,
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) # deceive the frontend for aten selections
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r._tensor = tensor
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if placeholder:
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if name is None:
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name = "input"
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r._node = graph.create_node(
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"placeholder", "placeholder", (graph._root,), name=namespace.create_name(name, tensor)
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)
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# ...the real tensor is held as an element on the tensor.
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if not r._tensor.is_meta:
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r._tensor = r._tensor.to(torch.device("meta"))
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return r
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@classmethod
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def __torch_dispatch__(cls, func, types, args=(), kwargs=None):
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def unwrap(x):
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nonlocal fake_device
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if isinstance(x, MetaProxy):
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fake_device = x.device
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x = x._tensor
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# assert not isinstance(x, MetaProxy)
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elif isinstance(x, torch.Tensor):
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fake_device = x.device
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x = x.to(torch.device("meta"))
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return x
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def get_node(x):
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if isinstance(x, torch.Tensor) and not hasattr(x, "_node"):
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x = MetaProxy(x, placeholder=True, name="weight")
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return x if not hasattr(x, "_node") else x._node
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args_node = tree_map(get_node, args)
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kwargs_node = tree_map(get_node, kwargs)
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node = graph.create_node("call_function", func, args_node, kwargs_node)
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if "device" in kwargs:
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fake_device = kwargs["device"]
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kwargs["device"] = torch.device("meta")
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args = tree_map(unwrap, args)
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kwargs = tree_map(unwrap, kwargs)
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# run aten for backend=CPU but actually on backend=Meta
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out = func(*args, **kwargs)
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# Now, we want to continue propagating this tensor, so we rewrap Tensors in
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# our custom tensor subclass
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def wrap(x):
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if isinstance(x, torch.Tensor):
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nonlocal fake_device
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if not x.is_meta:
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x = x.to(torch.device("meta"))
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return (
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MetaProxy(x, fake_device=fake_device)
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if isinstance(x, torch.Tensor) and not hasattr(x, "_tensor")
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else x
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)
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def set_node(x):
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x._node = node
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out = tree_map(wrap, out)
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tree_map(set_node, out)
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return out
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def wrap(x):
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return MetaProxy(x, fake_device=fake_device, placeholder=True) if isinstance(x, torch.Tensor) else x
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args = tree_map(wrap, args)
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kwargs = tree_map(wrap, kwargs)
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out = module(*args, **kwargs)
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for tensor in normalize_tuple(out):
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if is_autogradable(tensor) and tensor.requires_grad:
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grad = (
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torch.empty_like(tensor._tensor, device=torch.device("meta"))
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if isinstance(tensor, MetaProxy)
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else torch.empty_like(tensor, device=torch.device("meta"))
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)
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torch.autograd.backward(
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tensor, MetaProxy(grad, fake_device=tensor.device, placeholder=True), retain_graph=True
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)
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return graph
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