Add support for Tensorflow SparseTensors: Dot merging layer.

[hertschuh]

Added tf.SparseTensor support for ops:

• matmul (sparse support was very partial before this PR)
• squeeze
• expand_dims

Added tf.SparseTensor support for merging layer:

• Dot

[codecov]

Codecov Report

Patch coverage: 94.91% and project coverage change: +0.01% 🎉

Comparison is base (bf04179) 83.80% compared to head (e33e1cf) 83.82%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #937      +/-   ##
==========================================
+ Coverage   83.80%   83.82%   +0.01%     
==========================================
  Files         318      318              
  Lines       28780    28831      +51     
  Branches     5506     5518      +12     
==========================================
+ Hits        24120    24168      +48     
- Misses       3145     3146       +1     
- Partials     1515     1517       +2     

Flag	Coverage Δ
keras_core	83.71% <94.91%> (+0.01%)	⬆️
keras_core-jax	66.98% <15.25%> (-0.11%)	⬇️
keras_core-numpy	60.50% <15.25%> (-0.09%)	⬇️
keras_core-tensorflow	66.95% <94.91%> (+0.04%)	⬆️
keras_core-torch	69.05% <15.25%> (-0.11%)	⬇️

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Files Changed	Coverage Δ
keras_core/backend/tensorflow/numpy.py	93.93% <94.23%> (-0.03%)	⬇️
keras_core/ops/numpy.py	94.41% <100.00%> (+<0.01%)	⬆️

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[fchollet]

LGTM, thank you!