fix: Fix bug when targeting the TRT-LLM backend ensemble

[blongnv]

What does the PR do?

If a user follows the example code and tries to use the ensemble instead of tensorrt_llm_bls, they will get this error:

curl -s http://localhost:9000/v1/chat/completions -H 'Content-Type: application/json' -d '{"model": "ensemble", "messages": [{"role": "user", "content": "Say this is a test!"}]}' | jq
INFO:     127.0.0.1:40064 - "POST /v1/chat/completions HTTP/1.1" 400 Bad Request
{
  "detail": "Unknown backend"
}

After the PR fix, the response will be correct:

curl -s http://localhost:9000/v1/chat/completions -H 'Content-Type: application/json' -d '{"model": "ensemble", "messages": [{"role": "user", "content": "Say this is a test!"}]}' | jq
INFO:     127.0.0.1:47946 - "POST /v1/chat/completions HTTP/1.1" 200 OK
{
  "id": "cmpl-fefc9e2a-8a74-11ef-8e2a-0242c0a80102",
  "choices": [
    {
      "finish_reason": "stop",
      "index": 0,
      "message": {
        "content": "This is a test!",
        "tool_calls": null,
        "role": "assistant",
        "function_call": null
      },
      "logprobs": null
    }
  ],
  "created": 1728942057,
  "model": "ensemble",
  "system_fingerprint": null,
  "object": "chat.completion",
  "usage": null
}

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• closes GitHub issue: #xxx

[rmccorm4]

ref: DLIS-7451 to follow-up with a test for ensemble path to maintain support and catch regressions

[rmccorm4]

Pipeline: 19345468 ✅

[nnshah1]

@rmccorm4 should we check against a list of supported platform types here to rule out any accidents

[rmccorm4]

Sure, we can be strict here just in case.

There's currently no use case I'm aware of where the platform would be used to specify a backend (ex: platform: tensorrt_plan) that doesn't have a backend equivalent (other than ensemble), and we generally encourage use of backend for this purpose for pretty much everything other than ensembles - so I think it's fine to be strict for now.

[rmccorm4]

@blongnv could you modify it to be something like this?

# Explicitly handle ensembles to avoid any runtime validation errors
if not backend and model.config()["platform"] == "ensemble":
  backend = "ensemble"

[blongnv]

Implemented, please check