fix: Adding ensemble support for vllm container

[KrishnanPrash]

Currently, while building the vLLM container, we are not passing --backend=ensemble as one of the flags, which leads to lack of support when people try running ensemble models in the vLLM container.

To support this, adding --backend=ensemble to the build instructions should provide support going forward (>=24.10).
Additionally, adding a test case within the existing L0_vllm_backend job to catch lack of support in CI pipeline.

[rmccorm4]

Fixes triton-inference-server/server#7632

[rmccorm4]

This is going to break EXPECTED_NUM_TESTS=6.

You could update it to EXPECTED_NUM_TESTS=7, but I think it's easier to just append the ensemble to the other test that is testing model loading here:

vllm_backend/ci/L0_backend_vllm/vllm_backend/vllm_backend_test.py

Lines 55 to 59 in 78bf0fb

	# Load both vllm and add_sub models
	self.triton_client.load_model(self.vllm_load_test)
	self.assertTrue(self.triton_client.is_model_ready(self.vllm_load_test))
	self.triton_client.load_model(self.python_model_name)
	self.assertTrue(self.triton_client.is_model_ready(self.python_model_name))

ex:

        # Load both vllm and add_sub models 
	self.triton_client.load_model(self.vllm_load_test) 
	self.assertTrue(self.triton_client.is_model_ready(self.vllm_load_test)) 
	self.triton_client.load_model(self.python_model_name) 
	self.assertTrue(self.triton_client.is_model_ready(self.python_model_name)) 
        
        # Test to ensure that ensemble models are supported in vllm container.
        # If ensemble support is not enabled, triton will fail to load the ensemble.
	self.triton_client.load_model(self.ensemble_model_name) 
	self.assertTrue(self.triton_client.is_model_ready(self.ensemble_model_name)) 

Also use vllm_load_test model inside the ensemble instead of vllm_opt for the same reason.

[rmccorm4]

The tests documents some expected behavior around vllm_opt staying alive for the duration of the test:

vllm_backend/ci/L0_backend_vllm/vllm_backend/test.sh

Lines 53 to 57 in 78bf0fb

	# `vllm_opt` model will be loaded on server start and stay loaded throughout
	# unittesting. To test vllm model load/unload we use a dedicated
	# `vllm_load_test`. To ensure that vllm's memory profiler will not error out
	# on `vllm_load_test` load, we reduce "gpu_memory_utilization" for `vllm_opt`,
	# so that at least 60% of GPU memory was available for other models.

So I think it's best not to mess with it in a new test and make use of vllm_load_test instead.

[oandreeva-nv]

nice one!