Revert "[docs] start a new FSDP notes doc (pytorch#117323)"

This reverts commit 7f474da.

Reverted pytorch#117323 on behalf of https://github.com/awgu due to broke docs ([comment](pytorch#117323 (comment)))

Author: pytorchmergebot

docs/source/notes/fsdp.rst

@@ -128,8 +128,6 @@ class FullyShardedDataParallel(nn.Module, _FSDPState):
     .. _`Xu et al.`: https://arxiv.org/abs/2004.13336
     .. _DeepSpeed: https://www.deepspeed.ai/
 
-    For advanced notes please refer to :ref:`fsdp_notes`.
-
     Example::
 
         >>> # xdoctest: +SKIP("undefined variables")