Microbenchmarking matrix mult (einsum, torch.mult, torch.mm) (#63654)

Microbenchmarking matrix mult (einsum, torch.mult, torch.mm) (#63654)

Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/63654

Test Plan:
```
> buck run mode/opt caffe2/benchmarks/operator_benchmark/pt:matrix_mult_test

# ----------------------------------------
# PyTorch/Caffe2 Operator Micro-benchmarks
# ----------------------------------------
# Tag : short

# Benchmarking PyTorch: einsum_bmm
# Mode: Eager
# Name: einsum_bmm_B4_M5_N3_K2_cpu
# Input: B: 4, M: 5, N: 3, K: 2, device: cpu
Forward Execution Time (us) : 27.970

# Benchmarking PyTorch: einsum_bmm
# Mode: Eager
# Name: einsum_bmm_B32_M25_N20_K30_cpu
# Input: B: 32, M: 25, N: 20, K: 30, device: cpu
Forward Execution Time (us) : 41.830

# Benchmarking PyTorch: einsum_bmm
# Mode: Eager
# Name: einsum_bmm_B128_M100_N120_K110_cpu
# Input: B: 128, M: 100, N: 120, K: 110, device: cpu
Forward Execution Time (us) : 499.114

# Benchmarking PyTorch: bmm
# Mode: Eager
# Name: bmm_B4_M5_N3_K2_cpu
# Input: B: 4, M: 5, N: 3, K: 2, device: cpu
Forward Execution Time (us) : 6.268

# Benchmarking PyTorch: bmm
# Mode: Eager
# Name: bmm_B32_M25_N20_K30_cpu
# Input: B: 32, M: 25, N: 20, K: 30, device: cpu
Forward Execution Time (us) : 12.676

# Benchmarking PyTorch: bmm
# Mode: Eager
# Name: bmm_B128_M100_N120_K110_cpu
# Input: B: 128, M: 100, N: 120, K: 110, device: cpu
Forward Execution Time (us) : 438.219

# Benchmarking PyTorch: einsum_elementwise
# Mode: Eager
# Name: einsum_elementwise_B4_M5_N3_cpu
# Input: B: 4, M: 5, N: 3, device: cpu
Forward Execution Time (us) : 7.657

# Benchmarking PyTorch: einsum_elementwise
# Mode: Eager
# Name: einsum_elementwise_B32_M25_N20_cpu
# Input: B: 32, M: 25, N: 20, device: cpu
Forward Execution Time (us) : 18.523

# Benchmarking PyTorch: einsum_elementwise
# Mode: Eager
# Name: einsum_elementwise_B100_M90_N110_cpu
# Input: B: 100, M: 90, N: 110, device: cpu
Forward Execution Time (us) : 55.103

# Benchmarking PyTorch: mul
# Mode: Eager
# Name: mul_B4_M5_N3_cpu
# Input: B: 4, M: 5, N: 3, device: cpu
Forward Execution Time (us) : 2.501

# Benchmarking PyTorch: mul
# Mode: Eager
# Name: mul_B32_M25_N20_cpu
# Input: B: 32, M: 25, N: 20, device: cpu
Forward Execution Time (us) : 10.589

# Benchmarking PyTorch: mul
# Mode: Eager
# Name: mul_B100_M90_N110_cpu
# Input: B: 100, M: 90, N: 110, device: cpu
Forward Execution Time (us) : 50.102

Reviewed By: ajyu

Differential Revision: D30455179

fbshipit-source-id: 9f2d92b2d2b860f41a8e59be2cc086d75b587f7b