Fix release action

[thisac]

Context: Automatic PyPI release action is failing due to failing tests when simply running tests with pytest tests (instead of make tests).

Description of the Change:

• Adds a random seed to the tests run on the upload action.
• Sets the env var SYMPY_USE_CACHE to "no" to avoid issue with sympy caching (see #684 for details).
• Fixes tolerances in a few tests that were failing too often (specifically when running without setting a seed).
• Replaces _einsum_v1 with tf.einsum in all places but one (still an issue with sf.backends.tfbackend.ops.conditional_state() which I wasn't able to solve) and also removed the einsum test marked XFAIL (was XPASSing).

Benefits:

• It should now be possible to run all tests directly using pytest tests.
• The automatic PyPI release on GitHub published should now work.

Possible Drawbacks: None

Related GitHub Issues: None

[codecov[bot]]

Codecov Report

Merging #722 (606a50b) into master (d2d5a3c) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #722      +/-   ##
==========================================
+ Coverage   98.30%   98.33%   +0.02%     
==========================================
  Files          78       78              
  Lines        9614     9603      -11     
==========================================
- Hits         9451     9443       -8     
+ Misses        163      160       -3     

Impacted Files	Coverage Δ
strawberryfields/backends/tfbackend/circuit.py	96.62% <ø> (+0.18%)	:arrow_up:
strawberryfields/backends/tfbackend/states.py	98.97% <ø> (+0.48%)	:arrow_up:
strawberryfields/backends/tfbackend/ops.py	98.27% <100.00%> (+0.12%)	:arrow_up:
strawberryfields/decompositions.py	99.44% <100.00%> (ø)

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