Several linalg functions support

[fbadine]

As part of mlx backend support (#19571), this PR adds support for the following functions in linalg.py:

• lu_factor (supported no in mlx)
• solve_triangular (currently supported in mlx)
• lstsq (implemented the same way as TensorFlow's one was implemented)
• eig (used numpy's implementation until it is supported on mlx)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 39 lines in your changes missing coverage. Please review.

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Files with missing lines	Patch %	Lines
keras/src/backend/mlx/linalg.py	0.00%	39 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##             mlx   #21260   +/-   ##
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  Coverage       ?   75.79%           
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  Files          ?      577           
  Lines          ?    57954           
  Branches       ?     8998           
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  Hits           ?    43925           
  Misses         ?    11995           
  Partials       ?     2034           

Flag	Coverage Δ
keras	75.61% <0.00%> (?)
keras-jax	60.70% <0.00%> (?)
keras-numpy	56.11% <0.00%> (?)
keras-tensorflow	61.00% <0.00%> (?)
keras-torch	59.95% <0.00%> (?)

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[fchollet]

LGTM - Nice work!