Added scale parameterization to Exponential

[manulpatel]

What is this PR about?
Fixes #6637
Adds alternative scale parametrization to exponential function. scale = 1 / lam

Checklist

• Explain important implementation details 👆
• Make sure that the pre-commit linting/style checks pass.
• Link relevant issues (preferably in nice commit messages)
• Are the changes covered by tests and docstrings?
• Fill out the short summary sections 👇

Major / Breaking Changes

• ...

New features

• Extended functionality of specifing alternative scale parameter in exponential.

Bugfixes

• ...

Documentation

• ...

Maintenance

• Relavant test cases added in test_continuous.py.

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📚 Documentation preview 📚: https://pymc--6677.org.readthedocs.build/en/6677/

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[ricardoV94]

Thanks for opening the PR. I left some comments below

[codecov]

Codecov Report

Merging #6677 (2e413cf) into main (fa82fef) will decrease coverage by 11.55%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6677       +/-   ##
===========================================
- Coverage   89.50%   77.95%   -11.55%     
===========================================
  Files          94       95        +1     
  Lines       15942    16009       +67     
===========================================
- Hits        14269    12480     -1789     
- Misses       1673     3529     +1856     

Impacted Files	Coverage Δ
pymc/distributions/continuous.py	97.72% <100.00%> (+0.01%)	⬆️
pymc/logprob/basic.py	84.74% <100.00%> (-14.41%)	⬇️

... and 58 files with indirect coverage changes

[ricardoV94]

Thanks @manulpatel, will merge once tests pass