Allow to use custom handlers from pip packages

[hal-314]

First of all, many thanks for developing TorchServe!

Is your feature request related to a problem? Please describe.

You need to specify a file to use a custom handler.

This requirements prevents for code reusing. For example, your custom handler can't extend other custom handlers. Also, helper classes or custom transforms must be defined in the same file and can't be reused by other handlers.

Describe the solution

Allow to use handlers implemented in a Pypi package. So, torchserve --handler you allow to pass the import route to the handler. The value would need to be a valid import route that can be dynamically loaded with importlib library.

Example: torchserve --handler my_package.hander.custom_handler.CustomHandler

Describe alternatives solution

There are two alternative solutions:

• Do not use pypi package use --extra-files to pass extra python files. See this comment. However, user must be careful on adding of necessary files.
• Use a pypi package and pass to torchserve --handler handler file path as usual. This is how I do it currently.