deprecate gradient_transform

[timmysilv]

Context:
after #4774 was merged, @albi3ro realized we should have deprecated gradient_transform as well.

Description of the Change:
Deprecate gradient_transform which is no longer used

Benefits:
gradient_transform is closer to being removed.

Possible Drawbacks:
N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3b2caab) 99.64% compared to head (69ae057) 99.64%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4794      +/-   ##
==========================================
- Coverage   99.64%   99.64%   -0.01%     
==========================================
  Files         381      381              
  Lines       34302    34048     -254     
==========================================
- Hits        34181    33926     -255     
- Misses        121      122       +1     

Files	Coverage Δ
pennylane/gradients/gradient_transform.py	100.00% <ø> (ø)
pennylane/transforms/batch_transform.py	96.03% <100.00%> (+0.03%)	⬆️
pennylane/transforms/compile.py	100.00% <ø> (ø)
pennylane/transforms/op_transforms.py	100.00% <100.00%> (ø)
pennylane/transforms/qfunc_transforms.py	90.74% <100.00%> (+0.74%)	⬆️

... and 42 files with indirect coverage changes

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[timmysilv]

gradient_transform is not even rendered... it was silently removed already