Support custom mapping file

[jithunnair-amd]

Some projects (such as Triton?) using hipify-torch would benefit from the ability to provide a custom cuda_to_hip_mapping file to be able to:

1. Specify CUDA-->HIP API mappings which are currently not present in the hipify-torch mappings. This would typically be a short-term fix until those mappings are integrated into hipify-torch.
2. Specify mappings of project-specific classes/datatypes etc. in case it is preferred to have different naming for the ROCm/HIP build of the project

This feature would be easier to provide once the json config file feature is added to hipify-torch.