BugFix: Tensorflow type promotion error with Tensors

[astralcai]

Context:
default.mixed used with tensorflow throws an error when applying a state that is not initialized in complex128, originally found here: https://discuss.pennylane.ai/t/error-with-keraslayer-and-amplitude-embedding/3924

Description of the Change:
tf.convert_to_tensor does not work for states that are already Tensor objects with a different dtype. Use tf.cast instead when this is the case.

Benefits:
BugFix

Related GitHub Issues:
#5085

Shortcut
[sc-55061]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d5e3cca) 99.69% compared to head (24e71a8) 99.68%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5155      +/-   ##
==========================================
- Coverage   99.69%   99.68%   -0.01%     
==========================================
  Files         394      394              
  Lines       36118    35850     -268     
==========================================
- Hits        36007    35738     -269     
- Misses        111      112       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.