Refactor common into cpu specific and truly common (#1817)

* refactor

* fix extension example

* fix no-cpu

mlx/backend/cpu/gemms/cblas.cpp

@@ -0,0 +1,44 @@
+// Copyright © 2025 Apple Inc.
+
+#include "mlx/backend/common/utils.h"
+#include "mlx/backend/cpu/gemm.h"
+#include "mlx/backend/cpu/lapack.h"
+
+namespace mlx::core {
+
+template <>
+void matmul<float>(
+    const array& a,
+    const array& b,
+    array& out,
+    bool a_transposed,
+    bool b_transposed,
+    size_t lda,
+    size_t ldb,
+    float alpha,
+    float beta) {
+  size_t M = a.shape(-2);
+  size_t N = b.shape(-1);
+  size_t K = a.shape(-1);
+
+  for (int i = 0; i < (a.size() / (M * K)); ++i) {
+    cblas_sgemm(
+        CblasRowMajor,
+        a_transposed ? CblasTrans : CblasNoTrans, // transA
+        b_transposed ? CblasTrans : CblasNoTrans, // transB
+        M,
+        N,
+        K,
+        alpha, // alpha
+        a.data<float>() + elem_to_loc(M * K * i, a.shape(), a.strides()),
+        lda,
+        b.data<float>() + elem_to_loc(K * N * i, b.shape(), b.strides()),
+        ldb,
+        beta, // beta
+        out.data<float>() + M * N * i,
+        out.shape(-1) // ldc
+    );
+  }
+}
+
+} // namespace mlx::core