fix: Use default model name when model name is None

[saimidu]

Issue #, if available:

Description of changes:
The model_name parameter when making a non-MME inference request, in the presence of a user's inference script, is treated as None by the python service for SM inference. This means that the context passed to the inference handler does not have a valid model_name attribute.

Therefore, GRPC-based inference requests for a model cannot be programmatically configured, and must instead always be hardcoded in the user's inference script. This requirement makes it harder to compile a model, or to use it with SageMaker Inference Recommender to produce a Neo-compilation-compatible model.

This PR makes a change to tfs_utils.py to use the default_model_name, obtained from the TFS_DEFAULT_MODEL_NAME environment variable, as the final option in case the input model_name is None and the request attributes do not contain the model name.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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