Add a kwarg for num_processes in the transpiler

[Ca1eb3]

Summary

fixes #9365
Add a kwarg to the transpiler which allows for overriding the user configs number of processes. As per this comment #9365 (comment) the order in which number of processes should be determined is the following:

1. Keyword arguments given explicitly
2. An argument set by environment variable (i.e. QISKIT_NUM_PROCS in this case)
3. An argument read from a user's configuration file
4. The system default

Details and comments

This required modifications to the transpiler to serialize all of the instruction durations before running any of the circuits. I am unsure if this is the best way to go about serialization of the instruction durations and would welcome suggestions of a more straight forward way to achieve this. This also requires modifications to the parallel_map function to allow for None to be passed in as the number of processes and to take on the default specified by the user config or the environment variable.

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• Overall coverage decreased (-0.009%) to 89.525%

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Totals
Change from base Build 7632033511:	-0.009%
Covered Lines:	59568
Relevant Lines:	66538

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[mtreinish]

Thanks for opening this PR, I'm definitely in favor of adding this argument and this is a good start. I have some small concerns about the current implementation that I left an inline comment on. I think that we should be adding an equivalent num_processes kwarg to the PassManager.run method (which internally can call parallel_map) definition and leverage that from inside transpile.

[mtreinish]

This LGTM now, thanks for the quick update