reduce input shapes for matmul (#29587)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29587
as title
Test Plan:
```
buck run //caffe2/benchmarks/operator_benchmark/pt:matmul_test -- --iteration 1
Reviewed By: hl475
Differential Revision: D18436317
fbshipit-source-id: 564143edc3d4400bcfafa0da11b7479562661b0c
