Check that IMEX Channels requested in NVIDIA_IMEX_CHANNELS are valid channel indices

[elezar]

With #1267 support for the NVIDIA_IMEX_CHANNELS was added to JIT CDI spec generation. One issue is that the validity of the channel index is not checked explicitly. This means that passing NVIDIA_IMEX_CHANNELS=bogus will succeed the spec generation (although it should fail when trying to inject the non-existent device).

We should update the logic to return a reasonable error message if:

• The channel specified is not in the range [0, 2047]
• The underlying device node does NOT exist.

[elezar]

I have assigned this to v1.18.0, but it could also be moved to the next feature release.