Add Support for Weighted Inputs in flow.train

[astro-jingtao]

Description:

This PR introduces support for weighted inputs in the flow.train method, allowing users to provide a sample_weight parameter. Users can now call the method as follows:

flow.train(data, sample_weight=sample_weight)

Key details:

• If sample_weight is not provided or is set to None, the function will maintain its previous behavior.
• This feature is particularly beneficial for astronomical applications, such as utilizing a 1/V weighted galaxy sample.

Changes Made:

• Modified the flow.train implementation to accept an optional sample_weight argument.
• Updated relevant docstring to reflect the new functionality.

Please review the changes and consider incorporating this feature into the main branch. I believe it will enhance the utility of the flow.train function for users working with weighted data.