[Feature] Add make_tensordict() function

[sicong-huang]

Description

Implement new function make_tensordict() that can automatically create tensordict from keyword arguments. When batch_size is unspecified, it assumes maximum possible batch_size based on the input source dict.

Motivation and Context

Closes #518

Types of changes

What types of changes does your code introduce? Remove all that do not apply:

• New feature (non-breaking change which adds core functionality)
• Documentation (update in the documentation)

Checklist

Go over all the following points, and put an x in all the boxes that apply.
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• I have read the CONTRIBUTION guide (required)
• My change requires a change to the documentation.
• I have updated the tests accordingly (required for a bug fix or a new feature).
• I have updated the documentation accordingly.

[vmoens]

Brilliant! Just a couple of things and we'll be good to go

[vmoens]

LGTM thanks for this!