Add fastpath test for mask check flag (#82999)
Summary: Check that fastpath is taken, which type (sparsity fastpath or normal) for mask that is aligned and one that is not.
Test Plan: buck test caffe2/test:test_transformers
Differential Revision: D38259928
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82999
Approved by: https://github.com/jbschlosser
