Record git revision during the installation

[grwlf]

By this PR we introduce and auto-generated catalyst._revision file containing __revision__ variable. When possible, this variable will contain latest known Git revision. In particular, users will be able to report the exact revisions by doing import catalyst; print(catalyst.__revision__) before submitting issues.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.49%. Comparing base (57dbadf) to head (19125fd).

❗ Current head 19125fd differs from pull request most recent head 33d4cdf. Consider uploading reports for the commit 33d4cdf to get more accurate results

Additional details and impacted files

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  Coverage   99.49%   99.49%           
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  Files          50       50           
  Lines        8739     8745    +6     
  Branches      624      624           
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+ Hits         8695     8701    +6     
  Misses         25       25           
  Partials       19       19           

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[erick-xanadu]

@grwlf I like this! Do you think it would be a good idea to print this along with the version? Maybe a version() method? Along with the pennylane-lightning hash used to build the runtime?

[grwlf]

Do you think it would be a good idea to print this along with the version? Maybe a version() method? Along with the pennylane-lightning hash used to build the runtime?

@erick-xanadu I think it would be nice indeed, but it would require discussions and so I'd prefer to skip it for now because I think it is important to have at least something.

The investigation is required to find out the "most standard" solution. The __revision__ I am proposing resembles this PEP 8 standard. The standard does not mention any __revision__, but the version() you are suggesting departs from it in a different way. The closest (but still non-standard UPD: no, actually it is kind of compliant) way I know would be to set __version__ to something like 1.2.3.dev+gAAAAAAA (see here). But can we include the second git revision from PL/L in the same manner? 1.2.3.dev+gAAAAA+gBBBBB? Might be, not sure. I am ready initiate this discussion in the channel right after we merge this bare minimum PR.