Reduce binary size

[awni]

Reduce the binary size by moving the kernel dispatches to an outer level and adding back unsafe_weak_copy:

Binary size built with:

cmake .. \                                
  -DCMAKE_BUILD_TYPE=MinSizeRel \
  -DBUILD_SHARED_LIBS=ON \
  -DMLX_BUILD_CPU=ON \
  -DMLX_BUILD_SAFETENSORS=ON \
  -DMLX_BUILD_GGUF=OFF \
  -DMLX_METAL_JIT=ON

Pre: 35MB
Post: 11MB (potentially shrink more by updating a few more of the Primitives with the new pattern)

Benchmarks

These are some pretty lightweight CPU benchmarks to try and get at overhead. For anything larger the time differences aren't visible.

Bench	Pre	Post
MNIST	0.325 (s)	0.324 (s)
MNiST 10	0.525 (s)	0.543 (s)
No Compile MNIST	0.366 (s)	0.366 (s)
No Compile MNIST 10	0.750 (s)	0.818 (s)
Linear Regression	4027.99455 (it/s)	4659.03894 (it/s)
Logistic Regression	4056.80979 (it/s)	4079.13215 (it/s)

[angeloskath]

LGTM!