fix model reference train by excluding certian keys from camelization

Issue #, if available:

Description of changes: When initializing the JumpStartEstimator class, it throws an error while initializing the metric definitions

    847 for metric_definition in default_metric_definitions:
--> 848     if metric_definition["Name"] not in {
    849         definition["Name"] for definition in kwargs.metric_definitions
    850     }:
    851         kwargs.metric_definitions.append(metric_definition)

KeyError: 'Name'

I tried printing out the default_metric_definitions for the ModelReference and got the following:

[{'name': 'huggingface-textgeneration:eval-loss',
  'regex': "'eval_loss': ([0-9]+\\.[0-9]+)"},
 {'name': 'huggingface-textgeneration:train-loss',
  'regex': "'loss': ([0-9]+\\.[0-9]+)"}]

When I tried printing out the same for the base Model, I got this:

[{'Name': 'huggingface-textgeneration:eval-loss',
  'Regex': "'eval_loss': ([0-9]+\\.[0-9]+)"},
 {'Name': 'huggingface-textgeneration:train-loss',
  'Regex': "'loss': ([0-9]+\\.[0-9]+)"}]

Testing done:

Unit tests
Created TrainingJob locally with the existing code

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aws / sagemaker-python-sdk