[doc] add lazy init docs (#4808)

colossalai/lazy/lazy_init.py

@@ -472,30 +472,11 @@ class LazyTensor(torch.Tensor):
 class LazyInitContext:
     """Context manager for lazy initialization. Enables initializing the model without allocating real memory.
 
-    Usage:
-        1. The model is initialized, but no real memory is allocated.
-        >>> ctx = LazyInitContext()
-        >>> with ctx:
-        >>>     model = MyModel().cuda()
-
-        2. The model is initialized with ``MetaTensor`` as weights, but still no real memory is allocated.
-        >>> with ctx.traceable(model):
-        >>>     gm = symbolic_trace(model, meta_args=meta_args)
-        >>> # Solve the execution strategy and apply the strategy to the model
-        >>> strategy = StrategyAndSpec()
-
-        3. The model is initialized with ``torch.Tensor`` as weights, and real memory is allocated. (single device)
-        >>> model = ctx.materialize(model)
-
-        3. The model is initialized with sharded ``torch.Tensor`` as weights, and real memory is allocated. (distributed scenario)
-        >>> model = apply_strategy_to_all_params(model, strategy)
-        >>> model = ctx.distribute(model)
-
-    Warnings:
-        This API is still experimental and further modifications can be made to it.
-        For example:
-            1. Quantization strategies can be applied before allocating real memory.
-            2. Lazy initialization seems slower than normal initialization.
+    Args:
+        tensor_cls (Union[_MyTensor, LazyTensor], optional): This is only for test. Defaults to LazyTensor.
+        default_device (Optional[Union[torch.device, str, int]], optional): Defalt device for initialization.
+            If it's cuda, initilization will be accelerated, but cuda memory will be allocated. By default, it's cpu.
+            Defaults to None.
     """
 
     _replaced: bool = False

docs/sidebars.json

@@ -55,6 +55,7 @@
         },
         "features/pipeline_parallel",
         "features/nvme_offload",
+        "features/lazy_init",
         "features/cluster_utils"
       ]
     },

docs/source/en/basics/booster_api.md

@@ -32,6 +32,8 @@ Plugin is an important component that manages parallel configuration (eg: The ge
 
 More details about usages of each plugin can be found in chapter [Booster Plugins](./booster_plugins.md).
 
+Some plugins support lazy initialization, which can be used to save memory when initializating large models. For more details, please see [Lazy Initialization](../features/lazy_init.md).
+
 ### API of booster
 
 {{ autodoc:colossalai.booster.Booster }}

docs/source/en/features/lazy_init.md

+# Lazy initialization
+
+Author: [Hongxiu Liu](https://github.com/ver217)
+
+**Prerequisite:**
+- [Train with booster](../basics/booster_api.md)
+
+## Introduction
+
+Lazy initialization defers model initialization. It saves memory when initializing large models.
+
+If your model has `N` billion parameters and your memory (or GPU memory) is `M` GB, we recommend you use lazy initialization when `4N >= M`. Otherwise, it is optional.
+
+## Usage
+
+Lazy initialization must be used with booster.
+
+### API reference
+
+{{ autodoc:colossalai.lazy.LazyInitContext }}
+
+### Example
+
+```python
+import colossalai
+from colossalai.lazy import LazyInitContext
+from colossalai.booster import Booster
+from colossalai.booster.plugin import GeminiPlugin
+
+from transformers import LlamaForCausalLM, LlamaConfig, BertForPreTraining
+
+colossalai.launch({})
+plugin = GeminiPlugin()
+booster = Booster(plugin)
+
+# 1. Initialize model from scratch
+# Initialization on cuda will accelerate the initialization process but take more GPU memory.
+with LazyInitContext(default_device="cuda"):
+    model = LlamaForCausalLM(LlamaConfig(hidden_size=64, intermediate_size=172, num_hidden_layers=4, num_attention_heads=4))
+model, *_ = booster.boost(model)
+
+# 2. Initialize model from pretrained
+with LazyInitContext():
+    model = BertForPreTraining.from_pretrained("prajjwal1/bert-tiny")
+model, *_ = booster.boost(model)
+```
+
+> ⚠️ Lazy initialization from pretrained is supported for colossalai>0.3.3 or main branch.
+
+## Limitations
+
+As we claimed, lazy initialization must be used with booster. And only several plugins support it.
+
+| Plugin          | Supported | Remarks      |
+|-----------------|-----------|--------------|
+| Gemini          | Yes       |              |
+| Hybrid Parallel | Yes       |              |
+| Low Level Zero  | No        | No need      |
+| Torch DDP       | No        | Incompatible |
+| Torch FSDP      | No        | Incompatible |
+
+Not all models can be lazily initialized. In some cases, a part of parameters/buffers may be early initialized. But don't worry, this part usually takes a small proportion of the whole model.
+
+And some models are not supported at all which will raise an error. We tested models in torchvision, diffusers, timm, transformers, torchaudio and torchrec. Below models are not supported:
+
+| Model                         | Category     |
+|-------------------------------|--------------|
+| wav2vec2_base                 | torchaudio   |
+| hubert_base                   | torchaudio   |
+| ViTModel                      | transformers |
+| ViTForMaskedImageModeling     | transformers |
+| ViTForImageClassification     | transformers |
+| Blip2Model                    | transformers |
+| Blip2ForConditionalGeneration | transformers |
+
+<!-- doc-test-command: torchrun --standalone --nproc_per_node=2 lazy_iniy.py  -->

docs/source/zh-Hans/basics/booster_api.md

@@ -35,6 +35,8 @@ Booster 插件是管理并行配置的重要组件（eg：gemini 插件封装了
 
 若想了解更多关于插件的用法细节，请参考[Booster 插件](./booster_plugins.md)章节。
 
+有一些插件支持懒惰初始化，它能节省初始化大模型时的内存占用。详情请参考[懒惰初始化](../features/lazy_init.md)。
+
 ### Booster 接口
 
 <!--TODO: update autodoc -->

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

+# 懒惰初始化
+
+作者: [Hongxiu Liu](https://github.com/ver217)
+
+**前置教程:**
+- [Train with booster](../basics/booster_api.md)
+
+## 简介
+
+懒惰初始化延迟了模型的初始化。它能够节省在大模型初始化时的内存占用。
+
+如果你的模型有 `N` 十亿个参数并且你的内存（或显存）为 `M` GB, 我们推荐您在 `4N >= M` 时使用懒惰初始化。否则，懒惰初始化不是必须的。
+
+## 使用
+
+懒惰初始化必须与 booster 一起使用。
+
+### API 参考
+
+{{ autodoc:colossalai.lazy.LazyInitContext }}
+
+### 例子
+
+```python
+import colossalai
+from colossalai.lazy import LazyInitContext
+from colossalai.booster import Booster
+from colossalai.booster.plugin import GeminiPlugin
+
+from transformers import LlamaForCausalLM, LlamaConfig, BertForPreTraining
+
+colossalai.launch({})
+plugin = GeminiPlugin()
+booster = Booster(plugin)
+
+# 1. Initialize model from scratch
+# Initialization on cuda will accelerate the initialization process but take more GPU memory.
+with LazyInitContext(default_device="cuda"):
+    model = LlamaForCausalLM(LlamaConfig(hidden_size=64, intermediate_size=172, num_hidden_layers=4, num_attention_heads=4))
+model, *_ = booster.boost(model)
+
+# 2. Initialize model from pretrained
+with LazyInitContext():
+    model = BertForPreTraining.from_pretrained("prajjwal1/bert-tiny")
+model, *_ = booster.boost(model)
+```
+
+> ⚠️ 使用懒惰初始化加载预训练模型在 colossalai>0.3.3 或主分支上支持。
+
+## 限制
+
+我们提到，懒惰初始化必须与 booster 一起使用。只有几个插件支持它。
+
+| 插件            | 支持情况 | 备注   |
+|-----------------|---------|--------|
+| Gemini          | 是       |        |
+| Hybrid Parallel | 是       |        |
+| Low Level Zero  | 否       | 不需要 |
+| Torch DDP       | 否       | 不兼容 |
+| Torch FSDP      | 否       | 不兼容 |
+
+不是所有的模型都可以懒惰初始化。在某些情况下，一部分参数/缓冲区可能会被提前初始化。但是不用担心，这部分通常只占整个模型的一小部分。
+
+并且一些模型完全不支持，会引发错误。我们测试了 torchvision, diffusers, timm, transformers, torchaudio 和 torchrec 中的模型。以下模型不受支持：
+
+| 模型                          | 分类         |
+|-------------------------------|--------------|
+| wav2vec2_base                 | torchaudio   |
+| hubert_base                   | torchaudio   |
+| ViTModel                      | transformers |
+| ViTForMaskedImageModeling     | transformers |
+| ViTForImageClassification     | transformers |
+| Blip2Model                    | transformers |
+| Blip2ForConditionalGeneration | transformers |
+
+<!-- doc-test-command: torchrun --standalone --nproc_per_node=2 lazy_iniy.py  -->