[TensorIterator fixing mean to output correct result for half precisi… (#14878)

[TensorIterator fixing mean to output correct result for half precisi… (#14878)

Summary:
…on](#12115)

mean is calculated in two step sum()/numel(). For half precision, data gets
casted back to half after sum().
We fused the division into the reduction kernel by adding pre_op/post_op.

This allows us to do torch.ones(65536).cuda().half().mean() to return correct
result.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14878

Differential Revision: D13491159

Pulled By: soumith

fbshipit-source-id: e83802e1628b6d2615c45e18d7acf991d143a09e