[tfdbg2] Document the need to call set_soft_device_placement(True) on TPUs

PiperOrigin-RevId: 312785683
Change-Id: I388cb0a4d0d9eac1005bf4e52d10153d7bcd200f

tensorflow/python/debug/lib/check_numerics_callback.py

@@ -410,6 +410,21 @@ def enable_check_numerics(stack_height_limit=30,
      z = tf.matmul(y, y)
      ```
 
+  NOTE: If your code is running on TPUs, be sure to call
+  `tf.config.set_soft_device_placement(True)` before calling
+  `tf.debugging.enable_check_numerics()` as this API uses automatic outside
+  compilation on TPUs. For example:
+
+  ```py
+  tf.config.set_soft_device_placement(True)
+  tf.debugging.enable_check_numerics()
+
+  resolver = tf.distribute.cluster_resolver.TPUClusterResolver(tpu='')
+  strategy = tf.distribute.experimental.TPUStrategy(resolver)
+  with strategy.scope():
+    # ...
+  ```
+
   Args:
     stack_height_limit: Limit to the height of the printed stack trace.
       Applicable only to ops in `tf.function`s (graphs).

tensorflow/python/debug/lib/dumping_callback.py

@@ -721,6 +721,22 @@ def enable_dump_debug_info(dump_root,
   # Code to build, train and run your TensorFlow model...
   ```
 
+  NOTE: If your code is running on TPUs, be sure to call
+  `tf.config.set_soft_device_placement(True)` before calling
+  `tf.debugging.experimental.enable_dump_debug_info()` as this API uses
+  automatic outside compilation on TPUs. For example:
+
+  ```py
+  tf.config.set_soft_device_placement(True)
+  tf.debugging.experimental.enable_dump_debug_info(
+      logdir, tensor_debug_mode="FULL_HEALTH")
+
+  resolver = tf.distribute.cluster_resolver.TPUClusterResolver(tpu='')
+  strategy = tf.distribute.experimental.TPUStrategy(resolver)
+  with strategy.scope():
+    # ...
+  ```
+
   Args:
     dump_root: The directory path where the dumping information will be written.
     tensor_debug_mode: Debug mode for tensor values, as a string.