Add command line argument for sweeps

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Need to implement hyperparamter sweeps and hence need a command line argument for it.

**Describe the solution you'd like**
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Simple add a new sweeps sub-command to tang/main.py

**Describe alternatives you've considered**
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Not applicable.

**Additional context**
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This is a part of a larger project to enable tango to be able sweep hyerparameters: Hyperparameter sweeps #143