Make generated WidebandModulationsDataset samples independent of call order

[dustinlagoy]

First, thanks for all the work on this project. It is a great resource!!!

Is your feature request related to a problem? Please describe.
If I use the WidebandModulationsDataset to generate samples without using a DataLoader (or DatasetLoader) the samples depend on the order in which they are generated. For example:

data = WidebandModulationsDataset(...)
assert data[0] == data[0]

will fail because the generated sample at index 0 (or any index) changes each time you call data[0]. Both the number and characteristics of the generated signals and the added noise change on each subsequent call. This makes using on-the-fly generation of samples difficult unless one can ensure they are always generated in the same order.

Describe the solution you'd like
I think the dataset should generate the exact same sample for a given index regardless of any previous sample generation.

Describe alternatives you've considered
When generating samples to be written to disk, or training with on-the-fly generation of samples the data loader may ensure (it does at least for writing samples to disk) that the order of calls to WidebandModulationsDataset.__getitem__ are consistent and work around this issue. This may be sufficient for all practical use cases.

Additional context
I opened a pull request (#249) with sufficient changes to fix this issue. I understand if this feature is not desirable. In that case it may be nice to make this behavior clear in the documentation somewhere.

[MattCarrickPL]

Thanks for submitting this. We are discussing internally.