[shardformer] update seq parallel document (#4730)

* update doc of seq parallel

* fix typo

docs/source/en/features/shardformer.md

@@ -1,6 +1,6 @@
 # Shardformer
 
-Author: [Baizhou Zhang](https://github.com/Fridge003)
+Author: [Baizhou Zhang](https://github.com/Fridge003), [Bin Jia](https://github.com/FoolPlayer)
 
 **Prerequisite**
 - [Paradigms of Parallelism](../concepts/paradigms_of_parallelism.md)
@@ -16,7 +16,7 @@ Author: [Baizhou Zhang](https://github.com/Fridge003)
 - [GPipe: Efficient Training of Giant Neural Networks using Pipeline Parallelism](https://arxiv.org/abs/1811.06965)
 - [FlashAttention-2: Faster Attention with Better Parallelism and Work Partitioning](https://arxiv.org/abs/2307.08691)
 - [Sequence Parallelism: Long Sequence Training from System Perspective](https://arxiv.org/abs/2105.13120)
-
+- [Reducing Activation Recomputation in Large Transformer Models](https://arxiv.org/abs/2205.05198)
 
 ## Introduction
 
@@ -74,6 +74,18 @@ is an example on how to trigger `Shardformer` through calling Shardformer APIs.
     ```
     when training ChatGLM-2 with Shardformer, and initialize your model with these imported classes.
 
+### Sequence Parallelism
+
+Sequence parallelism in `Shardformer` is a little different from [this one](https://colossalai.org/docs/basics/configure_parallelization/#sequence-parallel) which focuses on ring attention. In `Shardformer`, sequence parallelism just use with 1D tensor parallelism to to further reduce the memory occupation of activations in computations.
+
+1. In normal [1D tensor parallel](https://colossalai.org/docs/features/1D_tensor_parallel), there are 2 communication operations, $g$ and $\vec{g}$, $g$ will do one time All-Reduce in backward to get all gradient from all the devices and $\vec{g}$ will do one time All-Reduce in forward to get whole outputs from all the device.
+
+2. When using sequence parallelism, $\vec{g}$ needs to do All-Gather to gather the inputs in sequence dimension during forward and Reduce-Scatter to splite the gradient during backward. $\vec{g}$ needs to do Reduce-Scatter to splite the output of row linear layer of tensor parallel to all devices in sequence dimension, and All-Gather to get the whole gradient during backward.
+
+3. The implementation of All-Reduce using NCCL adopts the `Ring All-Reduce` approach, which consists of a Reduce-Scatter operation and an All-Gather operation with equal costs. Therefore, compared to sequence parallelism and tensor parallelism, it does not introduce additional communication overhead.
+
+4. One important thing to note is that when using sequence parallelism with 'Column Linear' of tensor parallelism,, during the backward computation of gradients, the complete input needs to be obtained. During the forward pass, only the portion of the input that is split along the sequence dimension is retained, shape like $(batch, sequence_len/k, hidden_states)$. Therefore, an additional All-Gather operation is required to obtain the complete input for gradient computation. However, in the implementation, it is possible to overlap the gradient computation with the All-Gather communication operation, which would not introduce additional communication overhead (corresponding to the `enable_sequence_overlap` parameter in `Shardformer`).
+
 ## How Shardformer Works
 
 Generally, Shardformer works through the following four kinds of *replacements*:
@@ -100,6 +112,7 @@ As a result, the optimizer will only compute the states corresponding to these p
 All of these replacements are implemented with manually written policies and forward functions.
 If you want to delve deeper into the design of Shardformer or customize your own Shardformer policies, please refer to our [Shardformer development document](https://github.com/hpcaitech/ColossalAI/blob/main/colossalai/shardformer/README.md) and [pipeline parallelism design](https://github.com/hpcaitech/ColossalAI/discussions/4050) for more details.
 
+
 ## Supporting Information
 
 Model/Feature Compatibility Matrix:

docs/source/zh-Hans/features/shardformer.md

@@ -1,6 +1,6 @@
 # Shardformer
 
-Author: [Baizhou Zhang](https://github.com/Fridge003)
+Author: [Baizhou Zhang](https://github.com/Fridge003), [Bin Jia](https://github.com/FoolPlayer)
 
 **预备知识**
 - [并行技术](../concepts/paradigms_of_parallelism.md)
@@ -16,6 +16,7 @@ Author: [Baizhou Zhang](https://github.com/Fridge003)
 - [GPipe: Efficient Training of Giant Neural Networks using Pipeline Parallelism](https://arxiv.org/abs/1811.06965)
 - [FlashAttention-2: Faster Attention with Better Parallelism and Work Partitioning](https://arxiv.org/abs/2307.08691)
 - [Sequence Parallelism: Long Sequence Training from System Perspective](https://arxiv.org/abs/2105.13120)
+- [Reducing Activation Recomputation in Large Transformer Models](https://arxiv.org/abs/2205.05198)
 
 
 ## 简介
@@ -65,6 +66,19 @@ Shardformer的配置由类`ShardConfig`的参数控制：
     并且使用这些导入的类初始化模型。
 
 
+### 序列并行 Sequence Parallelism
+
+在`Shardformer`中，序列并行与[此处](https://colossalai.org/docs/basics/configure_parallelization/#sequence-parallel)稍有不同，后者侧重于ring attention。在`Shardformer`中，序列并行仅与1D张量并行一起使用，以进一步减少计算中activation的内存占用。
+
+1. 在普通的[1D张量并行](https://colossalai.org/docs/features/1D_tensor_parallel)中，有两个通信操作$g$和$\vec{g}$，$g$在反向传播中进行一次全局归约以获取来自所有设备的梯度，而$\vec{g}$在正向传播中进行一次All-Reduce以获取来自所有设备的输出。
+
+2. 当使用序列并行时，$\vec{g}$需要在正向传播过程中进行All-Gather以获取序列维度上的输入，并在反向传播过程中进行Reduce-Scatter以分割梯度。$\vec{g}$需要进行Reduce-Scatter以将序列维度上的行线性层输出分割到所有设备上，并进行All-Gather以获取完整的梯度。
+
+3. 使用NCCL的All-reduce实现采用了`Ring All-Reduce`方法，由一次Reduce-Scatter和一次All-Gather组成，两者的开销相等。因此，与序列并行和张量并行相比，它并不会引入额外的通信开销。
+
+4. 需要注意的一点是，在张量并行的 “Column Linear” 中进行序列并行时，梯度的反向计算过程中需要获取完整的输入。在前向传播过程中，仅保留沿序列维度分割的输入部分，张量的形状例如$(batch, sequence\_len/k, hidden\_states)$。因此，需要进行额外的全局收集操作以获取完整的输入进行梯度计算。但是，在实现中，可以将梯度计算与全局收集通信操作重叠，这不会引入额外的通信开销（对应`Shardformer`中的`enable_sequence_overlap`参数）。
+
+
 ## Shardformer的工作原理
 
 通常来说，Shardformer通过以下四种“替换”进行工作：