Handling GDR-capable providers not requested for memory registration

[Denis Maryin]

Hello aws_ofi_nccl maintainers,

For GDR-capable providers which do not request memory registration (i.e. provide FI_HMEM but not FI_MR_HMEM or FI_MR_LOCAL), there is an issue in current implementation.

The function register_mr_buffers() keeps mr_handle intact, and as it is set to NULL by caller, mr_handle will be NULL.
Later during ofi_iflush(), NULL mr_handle are being treat as an error condition, which leads to returning ncclSystemError. But having NULL here is normal if provider did not ask for memory registration.
Later during ofi_closeRecv() mr_handle is being passed to fi_close(), and in this case it is NULL, this lead to segfault.

To fix this the following pull request was created #81
Please consider merging it.

BRs,
Denis

[Rashika Kheria]

Thank you for contributing! Could you let us know about provider against which this PR has been tested?

[Denis Maryin]

Sure. We tested against PSM3 provider.

[Rashika Kheria]

Merged the changes. Resolving issue.