fix: TRTInterpreter output lacks return value

[gs-olive]

Description

• Fixes error causing PyTree failures during Dynamo tracing for new converters returning lists of Tensors

The following error is encountered when testing converters:

Traceback (most recent call last):
  File "~/python_virtual_environments/torch_trt_venv/lib/python3.10/site-packages/parameterized/parameterized.py", line 620, in standalone_func
    return func(*(a + p.args), **p.kwargs, **kw)
  File "~/TensorRT/py/torch_tensorrt/dynamo/converters/test/test_split.py", line 25, in test_split
    self.run_test(
  File "~/TensorRT/py/torch_tensorrt/dynamo/converters/test_utils.py", line 102, in run_test
    super().run_test(
  File "~/TensorRT/py/torch_tensorrt/dynamo/fx_ts_compat/tools/common_fx2trt.py", line 84, in run_test
    interpreter_result = interpreter.run(lower_precision=precision)
  File "~/TensorRT/py/torch_tensorrt/dynamo/fx_ts_compat/fx2trt.py", line 212, in run
    super().run()
  File "~/python_virtual_environments/torch_trt_venv/lib/python3.10/site-packages/torch/fx/interpreter.py", line 155, in run
    return self.module.graph.process_outputs(output_val) if enable_io_processing else output_val
  File "~/python_virtual_environments/torch_trt_venv/lib/python3.10/site-packages/torch/fx/graph.py", line 858, in process_outputs
    return self._codegen.process_outputs(out)
  File "~/python_virtual_environments/torch_trt_venv/lib/python3.10/site-packages/torch/fx/graph.py", line 620, in process_outputs
    return pytree.tree_unflatten(out, self.pytree_info.out_spec)
  File "~/python_virtual_environments/torch_trt_venv/lib/python3.10/site-packages/torch/utils/_pytree.py", line 301, in tree_unflatten
    raise ValueError(
ValueError: tree_unflatten(values, spec): `values` has length 1 but the spec refers to a pytree that holds 2 items (TreeSpec(tuple, None, [*,
  *])).

The TorchGen output verifier, invoked here, requires the outputs to be a list of values. The TRTInterpreter was not returning any values, causing the first error, then was returning tuple values, causing the second issue. The cast introduced in this PR resolves those.

Related to: #1828 (comment)

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• [ x ] My code follows the style guidelines of this project (You can use the linters)
• [ x ] I have performed a self-review of my own code
• [ x ] I have commented my code, particularly in hard-to-understand areas and hacks
• [ x ] I have made corresponding changes to the documentation
• [ x ] I have added tests to verify my fix or my feature
• [ x ] New and existing unit tests pass locally with my changes
• [ x ] I have added the relevant labels to my PR in so that relevant reviewers are notified

[narendasan]

Change seems fine but when should this get merged?

[gs-olive]

Since the change is small, it can wait for #2104 to avoid major merge conflicts.

[narendasan]

LGTM