change: Improve defaults handling in ModelTrainer

[benieric]

Issue #, if available: #5047 #5116

Description of changes:

• Add missing dependency for ModelTrainer causing import errors

• Improve defaults handling in ModelTrainer

• Align all training artifacts to be uploaded to the default s3 path for a training job - s3://<default_bucket>/<default_prefix>/<base_job_name>/<job_name>/

• Default handling of InputDataConfig - any inputs uploaded by ModelTrainer will be uploaded under the input directory in artifacts bucket for training job like below.

 default_bucket
   - default_bucket_prefix
    	- base_job_name
        	- base_job_name-<timestamp>
        		- input

• Default handling of OutputDataConfig - when provided a path like output_data_config.s3_output_path = f"s3://{default_bucket}/{default_bucket_prefix}/{base_job_name}, Output artifacts will appear in the artifacts bucket for the training job like. Platform will automatically <training_job_name>/output to the s3_output_path :

 default_bucket
   - default_bucket_prefix
    	- base_job_name
        	- base_job_name-<timestamp>
        		- output 

• Default handling of TensorBoardOutputConfig - when provided a path like tensor_board_output_config.s3_output_path = f"s3://{default_bucket}/{default_bucket_prefix}/{base_job_name}, TensorBoard output artifacts will appear in the artifacts bucket for the training job like below. Platform will automatically add <training_job_name>/tensorboard-output to the s3_output_path:

 default_bucket
   - default_bucket_prefix
    	- base_job_name
        	- base_job_name-<timestamp>
        		- tensorboard-output 

• Default handling of CheckpointConfig - when provided a path like checkpoint_config.s3_uri = f"s3://{default_bucket}/{default_bucket_prefix}/{base_job_name}/{base_job_name-<timestamp>}/checkpoints, checkpoints will be uploaded to the artifacts bucket like below. Notice that Platform respects the s3_uri and does not append extra paths in output.

 default_bucket
   - default_bucket_prefix
    	- base_job_name
        	- base_job_name-<timestamp>
        		- checkpoints

Testing done:

• Unit Tests + Manual testing

Merge Checklist

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• I have read the CONTRIBUTING doc
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Tests

• I have added tests that prove my fix is effective or that my feature works (if appropriate)
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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.