[Docs] add faq document (#1420)

* [Docs] add faq document

* Update docs/en/faq.md

Co-authored-by: Junjun2016 <hejunjun@sjtu.edu.cn>

* Update docs/zh_cn/faq.md

Co-authored-by: Junjun2016 <hejunjun@sjtu.edu.cn>

* modify readme

* modify index.rst

Co-authored-by: Junjun2016 <hejunjun@sjtu.edu.cn>

README.md

@@ -154,6 +154,8 @@ We also provide many [training tricks](docs/en/tutorials/training_tricks.md) for
 
 A Colab tutorial is also provided. You may preview the notebook [here](demo/MMSegmentation_Tutorial.ipynb) or directly [run](https://colab.research.google.com/github/open-mmlab/mmsegmentation/blob/master/demo/MMSegmentation_Tutorial.ipynb) on Colab.
 
+Please refer to [FAQ](docs/en/faq.md) for frequently asked questions.
+
 ## Citation
 
 If you find this project useful in your research, please consider cite:

README_zh-CN.md

@@ -153,6 +153,8 @@ MMSegmentation 是一个基于 PyTorch 的语义分割开源工具箱。它是 O
 
 同时，我们提供了 Colab 教程。你可以在[这里](demo/MMSegmentation_Tutorial.ipynb)浏览教程，或者直接在 Colab 上[运行](https://colab.research.google.com/github/open-mmlab/mmsegmentation/blob/master/demo/MMSegmentation_Tutorial.ipynb)。
 
+如果遇到问题，请参考 [常见问题解答](docs/zh_cn/faq.md)。
+
 ## 引用
 
 如果你觉得本项目对你的研究工作有所帮助，请参考如下 bibtex 引用 MMSegmentation。

docs/en/faq.md

@@ -0,0 +1,8 @@
+# Frequently Asked Questions (FAQ)
+
+We list some common troubles faced by many users and their corresponding solutions here. Feel free to enrich the list if you find any frequent issues and have ways to help others to solve them. If the contents here do not cover your issue, please create an issue using the [provided templates](https://github.com/open-mmlab/mmsegmentation/blob/master/.github/ISSUE_TEMPLATE/error-report.md/) and make sure you fill in all required information in the template.
+
+## How to know the number of GPUs needed to train the model
+
+- Infer from the name of the config file of the model. You can refer to the `Config Name Style` part of [Learn about Configs](https://github.com/open-mmlab/mmsegmentation/blob/master/docs/en/tutorials/config.md ). For example, for config file with name `segformer_mit-b0_8x1_1024x1024_160k_cityscapes.py`, `8x1` means training the model corresponding to it needs 8 GPUs, and the batch size of each GPU is 1.
+- Infer from the log file. Open the log file of the model and search `nGPU` in the file. The number of figures following `nGPU` is the number of GPUs needed to train the model. For instance, searching for `nGPU` in the log file yields the record `nGPU 0,1,2,3,4,5,6,7`, which indicates that eight GPUs are needed to train the model.

docs/en/index.rst

@@ -44,6 +44,7 @@ Welcome to MMSegmenation's documentation!
    :caption: Notes
 
    changelog.md
+   faq.md
 
 .. toctree::
    :caption: Switch Language

docs/zh_cn/faq.md

@@ -0,0 +1,8 @@
+# 常见问题解答（FAQ）
+
+我们在这里列出了使用时的一些常见问题及其相应的解决方案。 如果您发现有一些问题被遗漏，请随时提 PR 丰富这个列表。 如果您无法在此获得帮助，请使用 [issue模板](https://github.com/open-mmlab/mmsegmentation/blob/master/.github/ISSUE_TEMPLATE/error-report.md/ )创建问题，但是请在模板中填写所有必填信息，这有助于我们更快定位问题。
+
+## 如何获知模型训练时需要的显卡数量
+
+- 看模型的config文件的命名。可以参考[学习配置文件](https://github.com/open-mmlab/mmsegmentation/blob/master/docs/zh_cn/tutorials/config.md )中的`配置文件命名风格`部分。比如，对于名字为`segformer_mit-b0_8x1_1024x1024_160k_cityscapes.py`的config文件，`8x1`代表训练其对应的模型需要的卡数为8，每张卡中的batch size为1。
+- 看模型的log文件。点开该模型的log文件，并在其中搜索`nGPU`，在`nGPU`后的数字个数即训练时所需的卡数。比如，在log文件中搜索`nGPU`得到`nGPU 0,1,2,3,4,5,6,7`的记录，则说明训练该模型需要使用八张卡。

docs/zh_cn/index.rst

@@ -44,6 +44,7 @@
    :caption: 说明
 
    changelog.md
+   faq.md
 
 .. toctree::
    :caption: 语言切换