added default behavior if no streams and epoch_size specified

[snarayan21]

Description of changes:

Added default behavior for when epoch_size is specified but no streams given in StreamingDataset instantiation.

Issue was that specifying epoch_size without any streams triggered a ValueError in the apply_weights method of the Stream class because it didn't want choose to be specified for the whole epoch unless weighting relatively. But with just one stream, the epoch_size would only apply to one stream, so we would choose epoch_size elements over the single dataset per epoch.

Issue #, if available:

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[karan6181]

LGTM. Thank You!