Adding multisample feature along with testcases

[VijayVignesh1]

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

What does this PR do?

Fixes #317

PR review

Added support for multisample item.

Basically added a boolean parameter which creates a batch of sub samples for each sample, given a list of transform functions.

Sample code:

    def transform_fn_sq(x, *args, **kwargs):
        """A simple transform function that doubles the input."""
        return x * 2

    def transform_fn_add(x):
        """A simple transform function that adds 3 to the input."""
        return x + 3

    def transform_fn_identity(x):
        """A simple transform function that returns the input as is."""
        return x

    dataset = StreamingDataset(
        data_dir,
        cache_dir=str(cache_dir),
        shuffle=False,
        transform=[transform_fn_sq, transform_fn_add, transform_fn_identity],
        is_multisample=True,
    )

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in GitHub issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

[VijayVignesh1]

@tchaton @deependujha @bhimrazy Can you verify the approach once? I can then make changes to the README.