[Docs][MPS] Add mps environment variable table (#129008)

Pull Request resolved: https://github.com/pytorch/pytorch/pull/129008
Approved by: https://github.com/malfet
ghstack dependencies: #129006

docs/source/mps_environment_variables.rst

@@ -0,0 +1,39 @@
+.. _mps_environment_variables:
+
+MPS Environment Variables
+==========================
+
+**PyTorch Environment Variables**
+
+.. list-table::
+  :header-rows: 1
+
+  * - Variable
+    - Description
+  * - ``PYTORCH_DEBUG_MPS_ALLOCATOR``
+    - If set to ``1``, set allocator logging level to verbose.
+  * - ``PYTORCH_MPS_HIGH_WATERMARK_RATIO``
+    - High watermark ratio for MPS allocator. By default, it is set to 1.7.
+  * - ``PYTORCH_MPS_LOW_WATERMARK_RATIO``
+    - Low watermark ratio for MPS allocator. By default, it is set to 1.4 if the memory is unified and set to 1.0 if the memory is discrete.
+  * - ``PYTORCH_MPS_PREFER_METAL``
+    - If set to ``1``, force using metal kernels instead of using MPS Graph APIs. For now this is only used for matmul op.
+  * - ``PYTORCH_ENABLE_MPS_FALLBACK``
+    - If set to ``1``, full back operations to CPU when MPS does not support them.
+
+.. note::
+
+    **high watermark ratio** is a hard limit for the total allowed allocations
+
+    - `0.0` : disables high watermark limit (may cause system failure if system-wide OOM occurs)
+    - `1.0` : recommended maximum allocation size (i.e., device.recommendedMaxWorkingSetSize)
+    - `>1.0`: allows limits beyond the device.recommendedMaxWorkingSetSize
+
+    e.g., value 0.95 means we allocate up to 95% of recommended maximum
+    allocation size; beyond that, the allocations would fail with OOM error.
+
+    **low watermark ratio** is a soft limit to attempt limiting memory allocations up to the lower watermark
+    level by garbage collection or committing command buffers more frequently (a.k.a, adaptive commit).
+    Value between 0 to m_high_watermark_ratio (setting 0.0 disables adaptive commit and garbage collection)
+    e.g., value 0.9 means we 'attempt' to limit allocations up to 90% of recommended maximum
+    allocation size.

docs/source/torch_environment_variables.rst

@@ -21,6 +21,7 @@ If you find anything in this documentation that is missing, incorrect, or could
 
    threading_environment_variables
    cuda_environment_variables
+   mps_environment_variables
    debugging_environment_variables
    miscellaneous_environment_variables
    logging