[JIT] Use `is_buffer` in `BufferPolicy::valid` (#49588)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49588
**Summary**
`BufferPolicy::valid` uses `!typ->is_parameter(i)` to check if an
attribute is a buffer or not; it should use `type->is_buffer(i)` instead.
It also removes a forward compatibility gate in `python_print.cpp` that
has prevented the preservation of buffer metadata during serialization
in fbcode. Without this, the first change (to `BufferPolicy`) does not
work correctly in fbcode.
**Test Plan**
It is difficult to write an additional test that would have failed before this
commit because the two booleans `is_parameter` and `is_buffer` are never set
to `true` at the same time.
**Fixes**
This commit fixes #48746.
Test Plan: Imported from OSS
Reviewed By: xw285cornell
Differential Revision: D25633250
Pulled By: SplitInfinity
fbshipit-source-id: e727f8506f16d2e2b28f3d76a655f6528e7ac6cb
