Add custom ops ReplaceZero

[xadupre]

No description provided.

[wschin]

Please consider

1. format the code
2. add a few more details. (1) how to run this test, (2) kernel selection mechanism to explain why you define two kernels for fp16 and fp32.
3. avoid abbreviations as function name.

[wschin]

Please ping me when you want me to review again. Thanks.

[xadupre]

I chose cvt for convert, I thought it was a common abbrevation. I replaced by cast. For float32, float16, we can extend to bfloat16 if needed. I chose not to reduce compilation time but we definitly can add it. What do you mean by how to run the unit tests? There is one file associated to all the kernel implemented in that folder: test/cuda/test_cudaops.py. I just extended it with one of two models per kernel.

[wschin]

Thanks!