[nnc] Add an initialization expression to Reduce() (#53751)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/53751
Sometimes the initial value of a reduction expression needs to be
computed with reference to the loop axes; for example, adding bias can be
efficiently represented by initializing the accumulator from the bias tensor:
```
C[n, c, h, w] = bias[c]
for (...)
C[n, c, h, w] += ...
```
ghstack-source-id: 123592861
Test Plan: `buck test //caffe2/test/cpp/tensorexpr -- Reductions.InitFunction`
Reviewed By: navahgar
Differential Revision: D26940321
fbshipit-source-id: 8a08e19e5d0b9ad453a07fab8b61e75dcd3d626b
