[nnc] Speed up batchnorm benchmark
Summary:
Use better scheduling: fuse and parallelize NC, fuse and
vectorize HW.
```
-----------------------------------------------
N/C/H/W ATen NNC
-----------------------------------------------
1/64/112/112 45449 ns 36672 ns
1/256/14/14 15555 ns 7116 ns
1/128/28/28 15737 ns 8560 ns
1/64/56/56 20766 ns 12153 ns
1/512/7/7 16985 ns 8182 ns
5/64/112/112 2532475 ns 2069668 ns
5/256/14/14 24507 ns 12228 ns
5/128/28/28 29352 ns 20146 ns
5/64/56/56 44786 ns 38784 ns
5/512/7/7 22307 ns 20505 ns
```
Test Plan: benchmark results above
Reviewed By: navahgar
Differential Revision: D29288658
fbshipit-source-id: dd05efa4b7d26b6ad94f54a9ef6c8c47adb160b5
