ROIPool + Dispatcher + Autocast + Code Cleanup

[datumbox]

part of #2797 and #2796

Implements Dispatcher + Autocast for ROIPool.

I'm also doing some clean up on the original code such as:

• Remove primitive const declaration from method names.
• Using references when possible.

[codecov]

Codecov Report

Merging #2922 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2922   +/-   ##
=======================================
  Coverage   73.41%   73.41%           
=======================================
  Files          99       99           
  Lines        8801     8801           
  Branches     1389     1389           
=======================================
  Hits         6461     6461           
  Misses       1915     1915           
  Partials      425      425           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 1fe1e11...e8fba5e. Read the comment docs.

[fmassa]

Code looks great, thanks!

Can you add some tests for autocast as well?
We have an example of it for RoIAlign in

vision/test/test_ops.py

Lines 298 to 303 in 455cd57

	@unittest.skipIf(not torch.cuda.is_available(), "CUDA unavailable")
	def test_roi_align_autocast(self):
	for x_dtype in (torch.float, torch.half):
	for rois_dtype in (torch.float, torch.half):
	with torch.cuda.amp.autocast():
	self._test_forward(torch.device("cuda"), contiguous=False, x_dtype=x_dtype, rois_dtype=rois_dtype)

[fmassa]

Discussed with @datumbox offline, in order to avoid conflicts with some currently ongoing PRs he will be sending the tests in a separate PR

[fmassa]

Thanks a lot!