[inductor] scatter_reduce - skip .item() in backward if GradMode is not enabled (#107353)
Repeats #106429 for scatter_reduce so that the backward will pass for PT2. The .item() call is only needed to make double-backward work, which isn't supported anyway for PT2; so an easy fix is to just skip the .item() call if we know we won't need double-backward.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107353
Approved by: https://github.com/eellison
