Support bfloat16 on NULL backend (#12340)

* add failing test

* move test

* only run test with NULL default

* add skip reason

* add fix

.github/workflows/test.yml

@@ -264,7 +264,7 @@ jobs:
     - name: Run unit tests
       run: python -m pytest -n=auto test/unit/ --durations=20
     - name: Run targetted tests on NULL backend
-      run: NULL=1 python3 test/test_multitensor.py TestMultiTensor.test_data_parallel_resnet_train_step
+      run: NULL=1 python3 -m unittest test.test_multitensor.TestMultiTensor.test_data_parallel_resnet_train_step test/device/test_null.py
     - name: Run SDXL on NULL backend
       run: MAX_BUFFER_SIZE=0 NULL=1 DEBUG=1 python3 examples/sdxl.py --seed 0 --noshow --timing --fakeweights
     - name: Run Clip tests for SD MLPerf on NULL backend

test/device/test_null.py

@@ -0,0 +1,13 @@
+import unittest
+from tinygrad import dtypes, Device
+from tinygrad.device import is_dtype_supported
+
+@unittest.skipUnless(Device.DEFAULT=="NULL", "Don't run when testing non-NULL backends")
+class TestNULLSupportsDTypes(unittest.TestCase):
+  def test_null_supports_ints_floats_bool(self):
+    dts = dtypes.ints + dtypes.floats + (dtypes.bool,)
+    not_supported = [dt for dt in dts if not is_dtype_supported(dt, "NULL")]
+    self.assertFalse(not_supported, msg=f"expected these dtypes to be supported by NULL: {not_supported}")
+
+if __name__ == "__main__":
+  unittest.main()

tinygrad/device.py

@@ -327,8 +327,8 @@ def is_dtype_supported(dtype:DType, device:str|None=None) -> bool:
     if device == "METAL": return not CI
     if device in {"CUDA", "NV"}: return not CI and not getenv(f"{device}_PTX")
     if device in {"CPU"}: return not CI and platform.machine() in {"arm", "arm64", "aarch64", "x86_64", "amd64"}
-    return device in {"AMD", "PYTHON"}
-  if dtype in dtypes.fp8s: return device == "PYTHON"
+    return device in {"AMD", "PYTHON", "NULL"}
+  if dtype in dtypes.fp8s: return device in {"PYTHON", "NULL"}
   if device == "WEBGPU": return dtype in [dtypes.bool, dtypes.char, dtypes.uchar, dtypes.short,
                                           dtypes.ushort, dtypes.float, dtypes.int32, dtypes.uint32, dtypes.half]
   # for CI GPU and OSX, cl_khr_fp16 isn't supported