add METAL_FAST_MATH env var to disable metal fast math (#3369)

* env var METAL_FAST_MATH to disable fastmath for metal

use this to test impact of fast math. might need to disable compiler cache with DISABLE_COMPILER_CACHE

* failed onnx test with fast math

METAL_FAST_MATH=0 DISABLE_COMPILER_CACHE=1 NOOPT=1 python -m pytest -n=auto test/external/external_test_onnx_backend.py -k test_MaxPool3d_stride_padding_cpu

tinygrad/runtime/ops_metal.py

@@ -20,6 +20,7 @@ class MetalCompiler(Compiler):
       return subprocess.check_output(['xcrun', '-sdk', 'macosx', 'metallib', '-', '-o', '-'], input=air)
     else:
       options = Metal.MTLCompileOptions.new()
+      options.setFastMathEnabled_(getenv("METAL_FAST_MATH", 1))
       library = unwrap2(self.device.device.newLibraryWithSource_options_error_(src, options, None))
       return library.libraryDataContents().bytes().tobytes()