Passing Pipeline Variable for entry_point while using XGBoost Estimator in script mode fails

Describe the bug Passing Pipeline Variable (e.g. S3 URL) for entry_point while using XGBoost Estimator in script mode fails to generate a pipeline definition. As the estimator tries to parse the variable and fails at url parsing.

To reproduce

model_output_path = ParameterString(name="ModelOutputPath")

xgb_script_mode_estimator = XGBoost(
        entry_point=script_path,
        framework_version="1.5-1",  # Note: framework_version is mandatory
        role=role,
        instance_count=training_instance_count,
        instance_type=training_instance_type,
        output_path=model_output_path,
        hyperparameters={
            "eval_metric": eval_metric,
            "min_child_weight": min_child_weight
        },
        checkpoint_s3_uri=checkpoint_path
    )

    train_input = TrainingInput(
        s3_processed_training_samples_url, content_type=content_type
    )
    validation_input = TrainingInput(
        s3_processed_testing_samples_url, content_type=content_type
    )

    step_train = TrainingStep(
        name="TrainModel",
        estimator=xgb_script_mode_estimator,
        inputs={"train": train_input, "validation": validation_input},
    )
pipeline = get_pipeline(
    region=region,
    role=role,
    default_bucket=default_bucket,
    model_package_group_name=model_package_group_name,
    pipeline_name=pipeline_name
)

import json
json.loads(pipeline.definition())

generates the error

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-7d78ecac635c> in <module>
      1 import json
----> 2 json.loads(pipeline.definition())

/opt/conda/lib/python3.7/site-packages/sagemaker/workflow/pipeline.py in definition(self)
    299     def definition(self) -> str:
    300         """Converts a request structure to string representation for workflow service calls."""
--> 301         request_dict = self.to_request()
    302         request_dict["PipelineExperimentConfig"] = interpolate(
    303             request_dict["PipelineExperimentConfig"], {}, {}

/opt/conda/lib/python3.7/site-packages/sagemaker/workflow/pipeline.py in to_request(self)
     89             if self.pipeline_experiment_config is not None
     90             else None,
---> 91             "Steps": list_to_request(self.steps),
     92         }
     93 

/opt/conda/lib/python3.7/site-packages/sagemaker/workflow/utilities.py in list_to_request(entities)
     40     for entity in entities:
     41         if isinstance(entity, Entity):
---> 42             request_dicts.append(entity.to_request())
     43         elif isinstance(entity, StepCollection):
     44             request_dicts.extend(entity.request_dicts())

/opt/conda/lib/python3.7/site-packages/sagemaker/workflow/steps.py in to_request(self)
    324     def to_request(self) -> RequestType:
    325         """Updates the request dictionary with cache configuration."""
--> 326         request_dict = super().to_request()
    327         if self.cache_config:
    328             request_dict.update(self.cache_config.config)

/opt/conda/lib/python3.7/site-packages/sagemaker/workflow/steps.py in to_request(self)
    212     def to_request(self) -> RequestType:
    213         """Gets the request structure for `ConfigurableRetryStep`."""
--> 214         step_dict = super().to_request()
    215         if self.retry_policies:
    216             step_dict["RetryPolicies"] = self._resolve_retry_policy(self.retry_policies)

/opt/conda/lib/python3.7/site-packages/sagemaker/workflow/steps.py in to_request(self)
    101             "Name": self.name,
    102             "Type": self.step_type.value,
--> 103             "Arguments": self.arguments,
    104         }
    105         if self.depends_on:

/opt/conda/lib/python3.7/site-packages/sagemaker/workflow/steps.py in arguments(self)
    306         """
    307 
--> 308         self.estimator._prepare_for_training(self.job_name)
    309         train_args = _TrainingJob._get_train_args(
    310             self.estimator, self.inputs, experiment_config=dict()

/opt/conda/lib/python3.7/site-packages/sagemaker/estimator.py in _prepare_for_training(self, job_name)
   2660                 constructor if applicable.
   2661         """
-> 2662         super(Framework, self)._prepare_for_training(job_name=job_name)
   2663 
   2664         self._validate_and_set_debugger_configs()

/opt/conda/lib/python3.7/site-packages/sagemaker/estimator.py in _prepare_for_training(self, job_name)
    659                 self.code_uri = self.uploaded_code.s3_prefix
    660             else:
--> 661                 self.uploaded_code = self._stage_user_code_in_s3()
    662                 code_dir = self.uploaded_code.s3_prefix
    663                 script = self.uploaded_code.script_name

/opt/conda/lib/python3.7/site-packages/sagemaker/estimator.py in _stage_user_code_in_s3(self)
    699             kms_key = None
    700         elif self.code_location is None:
--> 701             code_bucket, _ = parse_s3_url(self.output_path)
    702             code_s3_prefix = "{}/{}".format(self._current_job_name, "source")
    703             kms_key = self.output_kms_key

/opt/conda/lib/python3.7/site-packages/sagemaker/s3.py in parse_s3_url(url)
     37     parsed_url = urlparse(url)
     38     if parsed_url.scheme != "s3":
---> 39         raise ValueError("Expecting 's3' scheme, got: {} in {}.".format(parsed_url.scheme, url))
     40     return parsed_url.netloc, parsed_url.path.lstrip("/")
     41 

/opt/conda/lib/python3.7/site-packages/sagemaker/workflow/entities.py in __str__(self)
     79     def __str__(self):
     80         """Override built-in String function for PipelineVariable"""
---> 81         raise TypeError("Pipeline variables do not support __str__ operation.")
     82 
     83     def __int__(self):

TypeError: Pipeline variables do not support __str__ operation.

Expected behavior The pipeline definition has to be generated.

Screenshots or logs N/A

System information A description of your system. Please provide:

• SageMaker Python SDK version: 2.87.0
• Framework name (eg. PyTorch) or algorithm (eg. KMeans): XGBoost
• Framework version: 1.5-1
• Python version: 3.7.10
• CPU or GPU: CPU
• Custom Docker image (Y/N): N

Additional context Add any other context about the problem here.