change dropout lowering in symbolic_script (#18375)

change dropout lowering in symbolic_script (#18375)

Summary:
Dropout is now eligible for fusion, and generated fused kernels are just as fast as dropout in ATen. Change its lowering in symbolic script so that it can actually be fused. Still special-cased for cuda, because without fusion this lowering is less efficient than current (bernoulli_ * input). Testing is covered by the test case that ailzhang added (test_dropout_cuda).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18375

Differential Revision: D14611938

Pulled By: soumith

fbshipit-source-id: 11b18f4784e6c9265e382a8f8deca7add8df3b37