[fx] Add matrix multiplication fusion pass (#50151)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50151
**Summary**
This commit adds a graph transformation pass that merges several matrix
multiplications that use the same RHS operand into one large matrix
multiplication. The LHS operands from all of the smaller matrix multiplications
are concatenated together and used as an input in the large matrix multiply,
and the result is split in order to obtain the same products as the original
set of matrix multiplications.
**Test Plan**
This commit adds a simple unit test with two matrix multiplications that share
the same RHS operand.
`python test/test_fx_experimental.py -k merge_matmul -v`
Test Plan: Imported from OSS
Reviewed By: ngimel
Differential Revision: D25809409
Pulled By: SplitInfinity
fbshipit-source-id: fb55c044a54dea9f07b71aa60d44b7a8f3966ed0
