Fix inplace operation in LSTM when using torch backend

[dugujiujian1999]

Fix #18958

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[gbaned]

Hi @dugujiujian1999 Can you please sign CLA. Thank you!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (ccc202a) 79.65% compared to head (4202615) 75.40%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19010      +/-   ##
==========================================
- Coverage   79.65%   75.40%   -4.26%     
==========================================
  Files         339      339              
  Lines       35466    35466              
  Branches     6979     6979              
==========================================
- Hits        28251    26742    -1509     
- Misses       5613     7176    +1563     
+ Partials     1602     1548      -54     

Flag	Coverage Δ
keras	75.28% <100.00%> (-4.23%)	⬇️
keras-jax	?
keras-numpy	55.78% <0.00%> (ø)
keras-tensorflow	63.20% <100.00%> (ø)
keras-torch	63.66% <100.00%> (ø)

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[dugujiujian1999]

@gbaned Ok. Signed. Thank you.

[fchollet]

LGTM, thanks for the fix!