onnxruntime

Optimize python overhead of APEX amp (#9447)

* optimize python overhead of _post_amp_backward

* overwrite apex amp's zero_grad for faster implementation

* move unscale_fp16_grads_into_fp32_grads into C++ impl

* improve the efficiency furthur, reducing 3.5ms to 1.7ms for unilm.

* unilm 1.7ms to 338us: 1). optimize python list <==> std::vector copy, 2). launch the kernels as long as num_elem reach thresh hold. This help reduce the CUDA idel time.

* refine the logic a bit after validating

Co-authored-by: Baiju Meswani <bmeswani@microsoft.com>