[ROCm][doc] add ROCm section for building from source (#53845)

Summary:
Instructions for compiling PyTorch from source for ROCm were missing now that PyTorch 1.8 announced beta support for ROCm.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/53845

Reviewed By: heitorschueroff

Differential Revision: D27237916

Pulled By: malfet

fbshipit-source-id: c8be92fd76ea8df7e9f6944c0036568189f58808

README.md

@@ -165,6 +165,12 @@ Other potentially useful environment variables may be found in `setup.py`.
 
 If you are building for NVIDIA's Jetson platforms (Jetson Nano, TX1, TX2, AGX Xavier), Instructions to install PyTorch for Jetson Nano are [available here](https://devtalk.nvidia.com/default/topic/1049071/jetson-nano/pytorch-for-jetson-nano/)
 
+If you want to compile with ROCm support, install
+- [AMD ROCm](https://rocmdocs.amd.com/en/latest/Installation_Guide/Installation-Guide.html) 4.0 and above installation
+- ROCm is currently supported only for Linux system.
+
+If you want to disable ROCm support, export environment variable `USE_ROCM=0`.
+Other potentially useful environment variables may be found in `setup.py`.
 
 #### Install Dependencies
 
@@ -175,7 +181,7 @@ conda install numpy ninja pyyaml mkl mkl-include setuptools cmake cffi typing_ex
 
 On Linux
 ```bash
-# Add LAPACK support for the GPU if needed
+# CUDA only: Add LAPACK support for the GPU if needed
 conda install -c pytorch magma-cuda110  # or the magma-cuda* that matches your CUDA version from https://anaconda.org/pytorch/repo
 ```
 
@@ -208,6 +214,11 @@ export CMAKE_PREFIX_PATH=${CONDA_PREFIX:-"$(dirname $(which conda))/../"}
 python setup.py install
 ```
 
+Note that if you are compiling for ROCm, you must run this command first:
+```bash
+python tools/amd_build/build_amd.py
+```
+
 Note that if you are using [Anaconda](https://www.anaconda.com/distribution/#download-section), you may experience an error caused by the linker:
 
 ```plaintext