[1.13] Fix functorch version docs (pytorch#87392)

The functorch docs build seems to use a build of PyTorch that is not the
RC binary to produce the documentation. This results in the version
looking like '1.13.0a??????'.

This PR just hard-codes the functorch version in the docs to be 1.13 in
the release branch.

Test Plan:
- check docs preview

functorch/docs/source/conf.py

@@ -90,7 +90,7 @@
 project = 'functorch'
 copyright = 'PyTorch Contributors'
 author = 'PyTorch Contributors'
-functorch_version = str(functorch.__version__)
+functorch_version = '1.13'
 
 # The version info for the project you're documenting, acts as replacement for
 # |version| and |release|, also used in various other places throughout the