[NNC] Add loop slicing transforms (#43854)
Summary:
Add new transforms `sliceHead` and `sliceTail` to `LoopNest`, for example:
Before transformation:
```
for x in 0..10:
A[x] = x*2
```
After `sliceHead(x, 4)`:
```
for x in 0..4:
A[x] = x*2
for x in 4..10:
A[x] = x*2
```
After `sliceTail(x, 1)`:
```
for x in 0..4:
A[x] = x*2
for x in 4..9:
A[x] = x*2
for x in 9..10:
A[x] = x*2
```
`sliceHead(x, 10)` and `sliceTail(x, 10)` is no-op.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43854
Test Plan: Tests are added in `test_loopnest.cpp`, the tests cover the basic transformations, and also tests the combination with other transformations such as `splitWithTail`.
Reviewed By: nickgg
Differential Revision: D23417366
Pulled By: cheng-chang
fbshipit-source-id: 06c6348285f2bafb4be3286d1642bfbe1ea499bf
