pipeline.py: create dummy inputs if pipeline begins with an input stage?

[jllanfranchi]

Normally, one would run flux -> osc -> etc. However, it's possible one would want to test osc -> etc. As of now, "dummy" inputs are only generated in the case that the user specifies --only-stage. Is it a good idea to accommodate the above scenario, or might that lead to the user running an invalid pipeline? Or should --dummy-inputs be another argument to pipeline.py script so that the user must explicitly choose that behavior, and then create these whether or not --only-stage is specified? Note that --only-stage for an inputs-stage will require the generation of dummy inputs, until such time as we can get inputs from a file (another TODO!).

[jllanfranchi]

Seems like Stage class should have a generic dummy_inputs method that does something simple (like ones in the shape of input_binning) but this can be overridden by each service to achieve something more useful if desired.

[jllanfranchi]

Note updates in logic in pipeline.py (as of PR #269) when --only-stage is invoked, whereby we now use random numbers for BinnedTensorTransform rather than ones, since using the latter was masking an underlying bug. This sort of logic might make most sense if ported to the BinnedTensorTransform class and, to get map names right and whatnot, some bits be put in the Stage class.

[LeanderFischer]

I don't think this case is really needed to be covered..