Add mem efficient backend flag (#87946)
# Summary
Add in a torch.backends.cuda flag and update context manager to pic between the three implementations of the scaled_dot_product_attention.
cc @cpuhrsch @jbschlosser @bhosmer @mikaylagawarecki
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87946
Approved by: https://github.com/cpuhrsch
