Metal shaders for memory efficient self attention on large sequences (#964)

* Metal shaders for efficient self attention on large sequences

Updated fast attention: GEMM-ified with Steel primitives
Uses flash attention 1 for scale correction

* more compiler silencing

* Address rebase issues

* Templatize kernel instantiation, revise cpu bindings

* Safer writes to output

* Permit batch size > 1

* Numerical fixes for sdpa self attention

* Re-enable test, remove unused variable

* add benchmarking script

* Disable sdpa prior to perf tuning, and simplify tests for per-patch CI

python/src/fast.cpp

@@ -135,7 +135,6 @@ void init_fast(nb::module_& parent_module) {
             v (array): Input values array.
             scale (float): Scale for queries (typically ``1.0 / sqrt(q.shape(-1)``)
             mask (array, optional): An additive mask to apply to the query-key scores.
-
         Returns:
             array: The output array.
       )pbdoc");