Make --serialized-file argument optional

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This issue is reported in aws/sagemaker-pytorch-inference-toolkit#86

Context

• torchserve version:
• torch-model-archiver version:
• torch version:
• torchvision version [if any]:
• torchtext version [if any]:
• torchaudio version [if any]:
• java version:
• Operating System and version:

Your Environment

• Installed using source? [yes/no]:
• Are you planning to deploy it using docker container? [yes/no]:
• Is it a CPU or GPU environment?:
• Using a default/custom handler? [If possible upload/share custom handler/model]:
• What kind of model is it e.g. vision, text, audio?:
• Are you planning to use local models from model-store or public url being used e.g. from S3 bucket etc.?
  [If public url then provide link.]:
• Provide config.properties, logs [ts.log] and parameters used for model registration/update APIs:
• Link to your project [if any]:

Expected Behavior

Current Behavior

Possible Solution

Steps to Reproduce

...

Failure Logs [if any]