[core][rdt] Register your own transport at runtime for RDT

[dayshah]

Description

Adding a new public api for registering a tensor transport for RDT at runtime. You just need to call register_tensor_transport with a name, a list of supported devices, and a class that implements the TensorTransportManager interface.

Adding documentation for this too.

Testing by just registering NIXL, NCCL, and GLOO at runtime with this API, and having all the existing tests for the transports pass.

[gemini-code-assist]

Code Review

This pull request introduces a new public API, register_tensor_transport, to allow registering custom tensor transports for RDT at runtime. This is a valuable feature for extensibility. The implementation refactors TensorTransportEnum to use strings for transport types and migrates existing transports (NIXL, NCCL, GLOO) to the new registration API. The documentation is also updated accordingly.

However, I've identified a couple of issues. The most significant is that ray.put and ray.get still contain hardcoded checks that only permit "NIXL" and "OBJECT_STORE", which undermines the goal of supporting custom transports. Additionally, there's some duplicated code for validating transport names across different files. My review provides specific feedback to address these points.