Allow gradient transforms applied to broadcasted tapes if the broadcasted parameter(s) are not trainable

[dwierichs]

Context:
In #4462 we discussed the differentiation of broadcasted tapes/QNodes with gradient transforms. Due to some technical debt in the code for JVPs and due to unclear desired behaviour, applying gradient transforms to such QNodes was disallowed in #4480.
However, this also disallows applying gradient transforms to broadcasted tapes for which the broadcasted parameter(s) is/are not trainable, even though the problems discussed in #4462 do not apply in this case.

Description of the Change:
This PR lifts the restriction of applying gradient transforms to broadcasted tapes for the case that no trainable parameters are broadcasted.
The problem described in this forum post can be resolved by this, for example.

Benefits:
Unlocks gradient transforms for broadcasted tapes/QNodes as long as the broadcasting is in non-trainable parameters (which is quite common for QML applications: You want to broadcast across data points rather than trainable parameters.).

Possible Drawbacks:
Usage of tape.trainable_params although we don't want to rely on this property too much in the future.

[github-actions]

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Please edit doc/releases/changelog-dev.md with:

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.66%. Comparing base (0747c0a) to head (8a74b67).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5452      +/-   ##
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- Coverage   99.67%   99.66%   -0.01%     
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  Files         404      406       +2     
  Lines       37852    37590     -262     
==========================================
- Hits        37728    37465     -263     
- Misses        124      125       +1     

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[albi3ro]

👍

[vincentmr]

Hi @dwierichs , looks good to me :)