Using `TFGPT2LMHeadModel.generate` with tf.distribute.TPUStrategy and tf.function
## High-level description
I was wondering what were the steps that one is supposed to use to be able to do call `strategy.run` on `model.generate` with tf.distribute.TPUStrategy.
Right now, I get an error message telling me that TPUStrategy doesn't support pure eager functions, so I tried to put `model.__call__` and `model.generate` in a `tf.function`, in such a way:
```python
model.__call__ = tf.function(model.__call__,     experimental_relax_shapes=True)
model.generate = tf.function(model.generate,     experimental_relax_shapes=True)
```
but it doesn't work, when trying to jit either of just one of these or both (or neither).

## Environment info
- `transformers` version: 4.5.1
- Platform: Ubuntu 20.04 LTS
- Python version: 3.8.5
- PyTorch version (GPU?): N/A
- Tensorflow version (GPU?): Tensorflow 2.5.0rc2
- Using GPU in script?: Using TPUs
- Using distributed or parallel set-up in script?: tf.distribute.TPUStrategy with 8 TPUs (single TPU host)

### Who can help
@Rocketknight1, maybe @LysandreJik 

## To reproduce

Steps to reproduce the behavior, run this very small script:

```python
import sys
import transformers
import tensorflow as tf

TPU_NAME = "" # This is dependant on your setup. This is not the problem. Works perfectly with other things.
MODEL_KEY = "distilgpt2"
EXAMPLE_SENTENCE = "I like pizza, because it's "

cr = tf.distribute.cluster_resolver.TPUClusterResolver.connect(tpu=TPU_NAME)
strategy = tf.distribute.TPUStrategy(cr)
with strategy.scope():
    model = transformers.TFGPT2LMHeadModel.from_pretrained(MODEL_KEY)
tokenizer = transformers.AutoTokenizer.from_pretrained(MODEL_KEY)
tokens = tokenizer.encode(EXAMPLE_SENTENCE, return_tensors="tf") 

# This doesn't work (See *Error Message 1*, in the next section)
strategy.run(model.generate, args=(tokens,))

# So we try to jit the __call__ function:
model.__call__ = tf.function(
    model.__call__,
    experimental_relax_shapes=True,
)

# The following also doesn't work (See *Error Message 2*, in the next section)
strategy.run(model.generate, args=(tokens,))

# We try jitting generate
model.generate = tf.function(
    model.generate,
    experimental_relax_shapes=True,
)

# Once more, it doesn't work (See *Error Message 3*, in the next section)
# Only jitting `model.generate` gives the same error.
strategy.run(model.generate, args=(tokens,))
```

## Error messages:
### Error message 1:
No method is jitted.
```
---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
<ipython-input-9-44e0c5fc714c> in <module>
----> 1 strategy.run(model.generate, args=(tokens,))

~/.local/lib/python3.8/site-packages/tensorflow/python/distribute/tpu_strategy.py in run(self, fn, args, kwargs, options)
    392       objects, or `Tensor`s (for example, if running on a single replica).
    393     """
--> 394     validate_run_function(fn)
    395 
    396     fn, args, kwargs = _maybe_partial_apply_variables(fn, args, kwargs)

~/.local/lib/python3.8/site-packages/tensorflow/python/distribute/tpu_strategy.py in validate_run_function(fn)
     98       and not isinstance(fn, function.ConcreteFunction) \
     99       and not (callable(fn) and isinstance(fn.__call__, def_function.Function)):
--> 100     raise NotImplementedError(
    101         "TPUStrategy.run(fn, ...) does not support pure eager "
    102         "execution. please make sure the function passed into "

NotImplementedError: TPUStrategy.run(fn, ...) does not support pure eager execution. please make sure the function passed into `strategy.run` is a `tf.function` or `strategy.run` is called inside a `tf.function` if eager behavior is enabled.
```

### Error message 2:
Only `model.__call__` is jitted. Same error as #1
```
---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
<ipython-input-9-44e0c5fc714c> in <module>
----> 1 strategy.run(model.generate, args=(tokens,))

~/.local/lib/python3.8/site-packages/tensorflow/python/distribute/tpu_strategy.py in run(self, fn, args, kwargs, options)
    392       objects, or `Tensor`s (for example, if running on a single replica).
    393     """
--> 394     validate_run_function(fn)
    395 
    396     fn, args, kwargs = _maybe_partial_apply_variables(fn, args, kwargs)

~/.local/lib/python3.8/site-packages/tensorflow/python/distribute/tpu_strategy.py in validate_run_function(fn)
     98       and not isinstance(fn, function.ConcreteFunction) \
     99       and not (callable(fn) and isinstance(fn.__call__, def_function.Function)):
--> 100     raise NotImplementedError(
    101         "TPUStrategy.run(fn, ...) does not support pure eager "
    102         "execution. please make sure the function passed into "

NotImplementedError: TPUStrategy.run(fn, ...) does not support pure eager execution. please make sure the function passed into `strategy.run` is a `tf.function` or `strategy.run` is called inside a `tf.function` if eager behavior is enabled.
```

### Error message 3:
This is when either just `model.generate` is jitted, or both are jitted.
```
WARNING:tensorflow:AutoGraph could not transform <bound method Socket.send of <zmq.sugar.socket.Socket object at 0x7fa6913fd640>> and will run it as-is.
Please report this to the TensorFlow team. When filing the bug, set the verbosity to 10 (on Linux, `export AUTOGRAPH_VERBOSITY=10`) and attach the full output.
Cause: module, class, method, function, traceback, frame, or code object was expected, got cython_function_or_method
To silence this warning, decorate the function with @tf.autograph.experimental.do_not_convert
WARNING: AutoGraph could not transform <bound method Socket.send of <zmq.sugar.socket.Socket object at 0x7fa6913fd640>> and will run it as-is.
Please report this to the TensorFlow team. When filing the bug, set the verbosity to 10 (on Linux, `export AUTOGRAPH_VERBOSITY=10`) and attach the full output.
Cause: module, class, method, function, traceback, frame, or code object was expected, got cython_function_or_method
To silence this warning, decorate the function with @tf.autograph.experimental.do_not_convert
Setting `pad_token_id` to 50256 (first `eos_token_id`) to generate sequence
WARNING:tensorflow:The parameters `output_attentions`, `output_hidden_states` and `use_cache` cannot be updated when calling a model.They have to be set to True/False in the config object (i.e.: `config=XConfig.from_pretrained('name', output_attentions=True)`).
WARNING:tensorflow:The parameter `return_dict` cannot be set in graph mode and will always be set to `True`.
WARNING:tensorflow:From /home/jules/.local/lib/python3.8/site-packages/tensorflow/python/ops/array_ops.py:5043: calling gather (from tensorflow.python.ops.array_ops) with validate_indices is deprecated and will be removed in a future version.
Instructions for updating:
The `validate_indices` argument has no effect. Indices are always validated on CPU and never validated on GPU.
---------------------------------------------------------------------------
OperatorNotAllowedInGraphError            Traceback (most recent call last)
<ipython-input-9-44e0c5fc714c> in <module>
----> 1 strategy.run(model.generate, args=(tokens,))

~/.local/lib/python3.8/site-packages/tensorflow/python/distribute/tpu_strategy.py in run(self, fn, args, kwargs, options)
    400     fn = autograph.tf_convert(fn, autograph_ctx.control_status_ctx())
    401     options = options or distribute_lib.RunOptions()
--> 402     return self.extended.tpu_run(fn, args, kwargs, options)
    403 
    404   def experimental_assign_to_logical_device(self, tensor, logical_device_id):

~/.local/lib/python3.8/site-packages/tensorflow/python/distribute/tpu_strategy.py in tpu_run(self, fn, args, kwargs, options)
   1426   def tpu_run(self, fn, args, kwargs, options=None):
   1427     func = self._tpu_function_creator(fn, options)
-> 1428     return func(args, kwargs)
   1429 
   1430   def _tpu_function_creator(self, fn, options):

~/.local/lib/python3.8/site-packages/tensorflow/python/eager/def_function.py in __call__(self, *args, **kwds)
    887 
    888       with OptionalXlaContext(self._jit_compile):
--> 889         result = self._call(*args, **kwds)
    890 
    891       new_tracing_count = self.experimental_get_tracing_count()

~/.local/lib/python3.8/site-packages/tensorflow/python/eager/def_function.py in _call(self, *args, **kwds)
    931       # This is the first call of __call__, so we have to initialize.
    932       initializers = []
--> 933       self._initialize(args, kwds, add_initializers_to=initializers)
    934     finally:
    935       # At this point we know that the initialization is complete (or less

~/.local/lib/python3.8/site-packages/tensorflow/python/eager/def_function.py in _initialize(self, args, kwds, add_initializers_to)
    761     self._graph_deleter = FunctionDeleter(self._lifted_initializer_graph)
    762     self._concrete_stateful_fn = (
--> 763         self._stateful_fn._get_concrete_function_internal_garbage_collected(  # pylint: disable=protected-access
    764             *args, **kwds))
    765 

~/.local/lib/python3.8/site-packages/tensorflow/python/eager/function.py in _get_concrete_function_internal_garbage_collected(self, *args, **kwargs)
   3048       args, kwargs = None, None
   3049     with self._lock:
-> 3050       graph_function, _ = self._maybe_define_function(args, kwargs)
   3051     return graph_function
   3052 

~/.local/lib/python3.8/site-packages/tensorflow/python/eager/function.py in _maybe_define_function(self, args, kwargs)
   3442 
   3443           self._function_cache.missed.add(call_context_key)
-> 3444           graph_function = self._create_graph_function(args, kwargs)
   3445           self._function_cache.primary[cache_key] = graph_function
   3446 

~/.local/lib/python3.8/site-packages/tensorflow/python/eager/function.py in _create_graph_function(self, args, kwargs, override_flat_arg_shapes)
   3277     arg_names = base_arg_names + missing_arg_names
   3278     graph_function = ConcreteFunction(
-> 3279         func_graph_module.func_graph_from_py_func(
   3280             self._name,
   3281             self._python_function,

~/.local/lib/python3.8/site-packages/tensorflow/python/framework/func_graph.py in func_graph_from_py_func(name, python_func, args, kwargs, signature, func_graph, autograph, autograph_options, add_control_dependencies, arg_names, op_return_value, collections, capture_by_value, override_flat_arg_shapes)
    997         _, original_func = tf_decorator.unwrap(python_func)
    998 
--> 999       func_outputs = python_func(*func_args, **func_kwargs)
   1000 
   1001       # invariant: `func_outputs` contains only Tensors, CompositeTensors,

~/.local/lib/python3.8/site-packages/tensorflow/python/eager/def_function.py in wrapped_fn(*args, **kwds)
    670         # the function a weak reference to itself to avoid a reference cycle.
    671         with OptionalXlaContext(compile_with_xla):
--> 672           out = weak_wrapped_fn().__wrapped__(*args, **kwds)
    673         return out
    674 

~/.local/lib/python3.8/site-packages/tensorflow/python/framework/func_graph.py in wrapper(*args, **kwargs)
    984           except Exception as e:  # pylint:disable=broad-except
    985             if hasattr(e, "ag_error_metadata"):
--> 986               raise e.ag_error_metadata.to_exception(e)
    987             else:
    988               raise

OperatorNotAllowedInGraphError: in user code:

    /home/jules/.local/lib/python3.8/site-packages/tensorflow/python/distribute/tpu_strategy.py:1448 replicated_fn  *
        result[0] = fn(*replica_args, **replica_kwargs)
    /home/jules/.local/lib/python3.8/site-packages/transformers/generation_tf_utils.py:399 generate  *
        output = self._generate_no_beam_search(
    /home/jules/.local/lib/python3.8/site-packages/transformers/generation_tf_utils.py:455 _generate_no_beam_search  *
        while cur_len < max_length:
    /home/jules/.local/lib/python3.8/site-packages/tensorflow/python/autograph/operators/control_flow.py:858 while_stmt
        _py_while_stmt(test, body, get_state, set_state, opts)
    /home/jules/.local/lib/python3.8/site-packages/tensorflow/python/autograph/operators/control_flow.py:952 _py_while_stmt
        while test():
    /home/jules/.local/lib/python3.8/site-packages/tensorflow/python/framework/ops.py:900 __bool__
        self._disallow_bool_casting()
    /home/jules/.local/lib/python3.8/site-packages/tensorflow/python/framework/ops.py:503 _disallow_bool_casting
        self._disallow_when_autograph_enabled(
    /home/jules/.local/lib/python3.8/site-packages/tensorflow/python/framework/ops.py:489 _disallow_when_autograph_enabled
        raise errors.OperatorNotAllowedInGraphError(

    OperatorNotAllowedInGraphError: using a `tf.Tensor` as a Python `bool` is not allowed: AutoGraph did convert this function. This might indicate you are trying to use an unsupported feature.
```
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