Clarified the type of the inputs to callback APIs

The callback APIs were migrated to use jax.Arrays for both inputs and outputs
in JAX 0.4.27.

PiperOrigin-RevId: 634473890

docs/_tutorials/external-callbacks.md

@@ -56,7 +56,7 @@ def f(x):
 result = f(2)
 ```
 
-This works by passing the runtime value represented by `y` back to the host process, where the host can print the value.
+This works by passing the runtime value of `y` as a CPU {class}`jax.Array` back to the host process, where the host can print it.
 
 (external-callbacks-flavors-of-callback)=
 ## Flavors of callback

jax/_src/callback.py

@@ -72,7 +72,14 @@ def pure_callback_impl(
     vectorized: bool,
 ):
   del sharding, vectorized, result_avals
-  cpu_device, *_ = jax.local_devices(backend="cpu")
+  try:
+    cpu_device, *_ = jax.local_devices(backend="cpu")
+  except RuntimeError as e:
+    raise RuntimeError(
+        "jax.pure_callback failed to find a local CPU device to place the"
+        " inputs on. Make sure \"cpu\" is listed in --jax_platforms or the"
+        " JAX_PLATFORMS environment variable."
+    ) from e
   args = jax.device_put(args, cpu_device)
   with jax.default_device(cpu_device):
     try:
@@ -262,9 +269,8 @@ def pure_callback(
   For more explanation, see `External Callbacks`_.
 
   ``pure_callback`` enables calling a Python function in JIT-ed JAX functions.
-  The input ``callback`` will be passed NumPy arrays in place of JAX arrays and
-  should also return NumPy arrays. Execution takes place on CPU, like any
-  Python+NumPy function.
+  The input ``callback`` will be passed JAX arrays placed on a local CPU, and
+  it should also return JAX arrays on CPU.
 
   The callback is treated as functionally pure, meaning it has no side-effects
   and its output value depends only on its argument values. As a consequence, it
@@ -357,7 +363,14 @@ def io_callback_impl(
     ordered: bool,
 ):
   del result_avals, sharding, ordered
-  cpu_device, *_ = jax.local_devices(backend="cpu")
+  try:
+    cpu_device, *_ = jax.local_devices(backend="cpu")
+  except RuntimeError as e:
+    raise RuntimeError(
+        "jax.io_callback failed to find a local CPU device to place the"
+        " inputs on. Make sure \"cpu\" is listed in --jax_platforms or the"
+        " JAX_PLATFORMS environment variable."
+    ) from e
   args = jax.device_put(args, cpu_device)
   with jax.default_device(cpu_device):
     try:

jax/_src/debugging.py

@@ -77,13 +77,20 @@ class OrderedDebugEffect(effects.Effect):
 def debug_callback_impl(*args, callback: Callable[..., Any],
                         effect: DebugEffect):
   del effect
-  cpu_device, *_ = jax.local_devices(backend="cpu")
+  try:
+    cpu_device, *_ = jax.local_devices(backend="cpu")
+  except RuntimeError as e:
+    raise RuntimeError(
+        "jax.debug.callback failed to find a local CPU device to place the"
+        " inputs on. Make sure \"cpu\" is listed in --jax_platforms or the"
+        " JAX_PLATFORMS environment variable."
+    ) from e
   args = jax.device_put(args, cpu_device)
   with jax.default_device(cpu_device):
     try:
       callback(*args)
     except BaseException:
-      logger.exception("jax.debug_callback failed")
+      logger.exception("jax.debug.callback failed")
       raise
   return ()