Use slow but safe formula for prod_backward (#81617)
prod performs a sync to test for zeros as the formula is substantially
simpler if there are no zeros, but this doesn't work for meta tensors.
The double backwards formula works great in all cases though!
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81617
Approved by: https://github.com/soulitzer
