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

* refactor

* fix extension example

* fix no-cpu

mlx/backend/cpu/make_compiled_preamble.ps1

@@ -0,0 +1,38 @@
+# This script generates a C++ function that provides the CPU
+# code for use with kernel generation.
+#
+# Copyright © 2024 Apple Inc.
+
+$OUTPUT_FILE = $args[0]
+$CL = $args[1]
+$SRCDIR = $args[2]
+
+# Get command result as array.
+$CONTENT = & $CL /std:c++17 /EP "/I$SRCDIR" /Tp "$SRCDIR/mlx/backend/cpu/compiled_preamble.h"
+# Remove empty lines.
+# Otherwise there will be too much empty lines making the result unreadable.
+$CONTENT = $CONTENT | Where-Object { $_.Trim() -ne '' }
+# Concatenate to string.
+$CONTENT = $CONTENT -join "`n"
+
+# Append extra content.
+$CONTENT = @"
+$($CONTENT)
+using namespace mlx::core;
+using namespace mlx::core::detail;
+"@
+
+# Convert each char to ASCII code.
+# Unlike the unix script that outputs string literal directly, the output from
+# MSVC is way too large to be embedded as string and compilation will fail, so
+# we store it as static array instead.
+$CHARCODES = ([System.Text.Encoding]::ASCII.GetBytes($CONTENT) -join ', ') + ', 0'
+
+$OUTPUT = @"
+const char* get_kernel_preamble() {
+  static char preamble[] = { $CHARCODES };
+  return preamble;
+}
+"@
+
+Set-Content -Path $OUTPUT_FILE -Value $OUTPUT