add get_available_backends (#6771)

* lol

* 1 less line lmfao

* something like this?

* comment

* pylint

* just iterator

* backends -> devices

---------

Co-authored-by: George Hotz <72895+geohot@users.noreply.github.com>

tinygrad/device.py

@@ -2,7 +2,7 @@ from __future__ import annotations
 import multiprocessing, decimal, statistics, random
 from dataclasses import dataclass, replace
 from collections import defaultdict
-from typing import List, Optional, Dict, Tuple, Any, cast, Protocol, Type
+from typing import List, Optional, Dict, Tuple, Any, cast, Protocol, Type, Iterator
 import importlib, inspect, functools, pathlib, os, ctypes, atexit, time, contextlib, array
 from tinygrad.helpers import SAVE_SCHEDULE, getenv, diskcache_get, diskcache_put, DEBUG, GlobalCounters, flat_mv, from_mv, ProfileLogger, PROFILE
 from tinygrad.dtype import DType, ImageDType
@@ -27,16 +27,17 @@ class _Device:
     return ret
   @property
   def default(self) -> Compiled: return self[self.DEFAULT]
+  def get_available_devices(self) -> Iterator[str]:
+    for device in ["METAL", "AMD", "NV", "CUDA", "QCOM", "GPU", "CLANG", "LLVM"]:
+      with contextlib.suppress(Exception): yield self[device].dname
   @functools.cached_property
   def DEFAULT(self) -> str:
     if (from_env:=next((d for d in self._devices if d not in ["DISK", "NPY"] and getenv(d) == 1), None)): return from_env
-    for device in ["METAL", "AMD", "NV", "CUDA", "QCOM", "GPU", "CLANG", "LLVM"]:
-      try:
-        if self[device]:
-          os.environ[device] = "1"   # we set this in environment for spawned children
-          return device
-      except Exception: pass
-    raise RuntimeError("no usable devices")
+    try:
+      device = next(self.get_available_devices())
+      os.environ[device] = "1"   # we set this in environment for spawned children
+      return device
+    except StopIteration as exc: raise RuntimeError("no usable devices") from exc
 Device = _Device()
 
 # **************** Buffer + Allocators ****************