docs: document PYTORCH_ROOT (#3981)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Documentation**
- Introduced a new environment variable `PYTORCH_ROOT` for specifying
the path to the PyTorch Python library. This variable is detected
automatically to support building against the latest PyTorch version
from PyPI.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Jinzhe Zeng <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

doc/install/install-from-source.md

@@ -144,6 +144,7 @@ One may set the following environment variables before executing `pip`:
 | DP_ENABLE_TENSORFLOW                                | 0, 1                  | 1                      | {{ tensorflow_icon }} Enable the TensorFlow backend.                                                                                                                                                                                                                                                                                                                                                                                                |
 | DP_ENABLE_PYTORCH                                   | 0, 1                  | 0                      | {{ pytorch_icon }} Enable customized C++ OPs for the PyTorch backend. PyTorch can still run without customized C++ OPs, but features will be limited.                                                                                                                                                                                                                                                                                               |
 | TENSORFLOW_ROOT                                     | Path                  | Detected automatically | {{ tensorflow_icon }} The path to TensorFlow Python library. By default the installer only finds TensorFlow under user site-package directory (`site.getusersitepackages()`) or system site-package directory (`sysconfig.get_path("purelib")`) due to limitation of [PEP-517](https://peps.python.org/pep-0517/). If not found, the latest TensorFlow (or the environment variable `TENSORFLOW_VERSION` if given) from PyPI will be built against. |
+| PYTORCH_ROOT                                        | Path                  | Detected automatically | {{ pytorch_icon }} The path to PyTorch Python library. By default, the installer only finds PyTorch under the user site-package directory (`site.getusersitepackages()`) or the system site-package directory (`sysconfig.get_path("purelib")`) due to the limitation of [PEP-517](https://peps.python.org/pep-0517/). If not found, the latest PyTorch (or the environment variable `PYTORCH_VERSION` if given) from PyPI will be built against.   |
 | DP_ENABLE_NATIVE_OPTIMIZATION                       | 0, 1                  | 0                      | Enable compilation optimization for the native machine's CPU type. Do not enable it if generated code will run on different CPUs.                                                                                                                                                                                                                                                                                                                   |
 | CMAKE_ARGS                                          | str                   | -                      | Additional CMake arguments                                                                                                                                                                                                                                                                                                                                                                                                                          |
 | &lt;LANG&gt;FLAGS (`<LANG>`=`CXX`, `CUDA` or `HIP`) | str                   | -                      | Default compilation flags to be used when compiling `<LANG>` files. See [CMake documentation](https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_FLAGS.html).                                                                                                                                                                                                                                                                                  |