Fix TestGaussianGateApplication tests

[heltluke]

Context:
Two tests are failing with ValueError: The matrix S is not symplectic.

Description of the Change:
Reduces the scale from 1.0 to 0.5 in the failing tests, i.e. S = tf.Variable(random_symplectic(num_mode, scale=0.5), dtype=tf.complex128)

Benefits:
Tests pass

Possible Drawbacks:
We're not testing what we should be testing?

Related GitHub Issues:
N/A

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.21%. Comparing base (9a9a352) to head (0449e0d).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #762      +/-   ##
==========================================
- Coverage   98.33%   98.21%   -0.12%     
==========================================
  Files          78       78              
  Lines        9603     9539      -64     
==========================================
- Hits         9443     9369      -74     
- Misses        160      170      +10     

see 74 files with indirect coverage changes

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