Update on "Header cleanup"

Mostly removing unnecessary includes so that TensorIterator.h can be
included from NativeFunctions.h without causing cycles.  There some
cases where I moved code around so that I didn't have to pull in other
unnecessary stuff.

Signed-off-by: Edward Z. Yang <[email protected]>

Differential Revision: [D25278030](https://our.internmc.facebook.com/intern/diff/D25278030)

[ghstack-poisoned]

Author: ezyang