torch_xla/csrc/tensor_methods.cpp:880 : Check failed: xla::ShapeUtil::Compatible(shapes.back(), tensor_shape) 
Author: mabdullah1994
Pull request: no

## Environment info
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     Don't forget to fill out the missing fields in that output! -->

- `transformers` version: 4.5.0
- Platform: Linux-4.19.112+-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.7.10
- PyTorch version (GPU?): 1.8.1+cu101 (False)
- Tensorflow version (GPU?): 2.4.1 (False)
- Using GPU in script?: TPU
- Using distributed or parallel set-up in script?:

### Who can help
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- benchmarks: @patrickvonplaten
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- text generation: @patrickvonplaten
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@patrickvonplaten

## Information

I am using BigBirdForSequenceClassification and BigBirdTokenizer for a simple text classification problem on Google Colab TPU:

The problem arises when using:
* [ ] my own modified scripts: (Script shared) If I use the BigBirdForSequenceClassification model, I start getting weird errors on TPU.

```
from pathlib import Path

def read_imdb_split(split_dir):
    split_dir = Path(split_dir)
    texts = []
    labels = []
    for label_dir in ["pos", "neg"]:
        for text_file in (split_dir/label_dir).iterdir():
            texts.append(text_file.read_text())
            labels.append(0 if label_dir is "neg" else 1)

    return texts, labels

train_texts, train_labels = read_imdb_split('aclImdb/train')
test_texts, test_labels = read_imdb_split('aclImdb/test')

train_texts, train_labels = read_imdb_split('aclImdb/train')
test_texts, test_labels = read_imdb_split('aclImdb/test')

from sklearn.model_selection import train_test_split
train_texts, val_texts, train_labels, val_labels = train_test_split(train_texts, train_labels, test_size=.2)

from transformers import BigBirdTokenizer
tokenizer = BigBirdTokenizer.from_pretrained('google/bigbird-roberta-base')

train_encodings = tokenizer(train_texts, truncation=True, padding=True)
val_encodings = tokenizer(val_texts, truncation=True, padding=True)
test_encodings = tokenizer(test_texts, truncation=True, padding=True)

import torch

class IMDbDataset(torch.utils.data.Dataset):
    def __init__(self, encodings, labels):
        self.encodings = encodings
        self.labels = labels

    def __getitem__(self, idx):
        item = {key: torch.tensor(val[idx]) for key, val in self.encodings.items()}
        item['labels'] = torch.tensor(self.labels[idx])
        return item

    def __len__(self):
        return len(self.labels)

train_dataset = IMDbDataset(train_encodings, train_labels)
val_dataset = IMDbDataset(val_encodings, val_labels)
test_dataset = IMDbDataset(test_encodings, test_labels)

from transformers import BigBirdForSequenceClassification, Trainer, TrainingArguments
import torch_xla.distributed.xla_multiprocessing as xmp
import torch_xla.core.xla_model as xm

def main():
  training_args = TrainingArguments(
      output_dir='./results',          # output directory
      num_train_epochs=1,              # total number of training epochs
      per_device_train_batch_size=1,  # batch size per device during training
      per_device_eval_batch_size=1,   # batch size for evaluation
      warmup_steps=500,                # number of warmup steps for learning rate scheduler
      weight_decay=0.01,               # strength of weight decay
      logging_dir='./logs',            # directory for storing logs
      logging_steps=10,
  )

  model = BigBirdForSequenceClassification.from_pretrained('google/bigbird-roberta-base')

  trainer = Trainer(
      model=model,                         # the instantiated 🤗 Transformers model to be trained
      args=training_args,                  # training arguments, defined above
      train_dataset=train_dataset,         # training dataset
      eval_dataset=val_dataset             # evaluation dataset
  )

  trainer.train()

def _mp_fn(index):
  main()

xmp.spawn(_mp_fn, args=(), nprocs=1, start_method='fork')
```

The tasks I am working on is:
* [ ] my own task or dataset: Using the IMDB Dataset for Text Classification

## To reproduce

Steps to reproduce the behavior:

1. Setup TPU-client on google Colab: !pip install cloud-tpu-client https://storage.googleapis.com/tpu-pytorch/wheels/torch_xla-1.8-cp37-cp37m-linux_x86_64.whl
2. Download the dataset:
  a. !wget http://ai.stanford.edu/~amaas/data/sentiment/aclImdb_v1.tar.gz
  b. !tar -xf aclImdb_v1.tar.gz
3. Execute the given script

<!-- If you have code snippets, error messages, stack traces please provide them here as well.
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```
RuntimeError                              Traceback (most recent call last)
<ipython-input-14-38fb8a22e1a3> in <module>()
----> 1 xmp.spawn(_mp_fn, args=(), nprocs=1, start_method='fork')

7 frames
/usr/local/lib/python3.7/dist-packages/torch_xla/distributed/xla_multiprocessing.py in spawn(fn, args, nprocs, join, daemon, start_method)
    384   pf_cfg = _pre_fork_setup(nprocs)
    385   if pf_cfg.num_devices == 1:
--> 386     _start_fn(0, pf_cfg, fn, args)
    387   else:
    388     return torch.multiprocessing.start_processes(

/usr/local/lib/python3.7/dist-packages/torch_xla/distributed/xla_multiprocessing.py in _start_fn(index, pf_cfg, fn, args)
    321   # environment must be fully setup before doing so.
    322   _setup_replication()
--> 323   fn(gindex, *args)
    324 
    325 

<ipython-input-12-0ed5b032dbf1> in _mp_fn(index)
     32 
     33 def _mp_fn(index):
---> 34   main()

<ipython-input-12-0ed5b032dbf1> in main()
     29   )
     30 
---> 31   trainer.train()
     32 
     33 def _mp_fn(index):

/usr/local/lib/python3.7/dist-packages/transformers/trainer.py in train(self, resume_from_checkpoint, trial, **kwargs)
   1099             self.control = self.callback_handler.on_epoch_begin(self.args, self.state, self.control)
   1100 
-> 1101             for step, inputs in enumerate(epoch_iterator):
   1102 
   1103                 # Skip past any already trained steps if resuming training

/usr/local/lib/python3.7/dist-packages/torch_xla/distributed/parallel_loader.py in __next__(self)
     32 
     33   def __next__(self):
---> 34     return self.next()
     35 
     36   def __len__(self):

/usr/local/lib/python3.7/dist-packages/torch_xla/distributed/parallel_loader.py in next(self)
     44       if self._mark_step_batch_count <= self._batches_yielded:
     45         self._batches_yielded = 0
---> 46         xm.mark_step()
     47       else:
     48         self._batches_yielded += 1

/usr/local/lib/python3.7/dist-packages/torch_xla/core/xla_model.py in mark_step()
    716   torch_xla._XLAC._xla_step_marker(
    717       torch_xla._XLAC._xla_get_default_device(), [],
--> 718       wait=xu.getenv_as('XLA_SYNC_WAIT', bool, False))
    719   # Only emit metrics from the first local device index, to avoid emitting the
    720   # same values from different threads.

RuntimeError: Error while lowering: s64[1,2368]{1,0} aten::copysign, pad=(0, 19, 0, 0), value=0
Error: /pytorch/xla/torch_xla/csrc/helpers.h:100 : Check failed: scalar_value.isIntegral() 
*** Begin stack trace ***
	tensorflow::CurrentStackTrace()
	torch_xla::XlaHelpers::ScalarValue(c10::Scalar, xla::PrimitiveType, xla::XlaBuilder*)
	
	torch_xla::ir::ops::ConstantPadNd::Lower(torch_xla::ir::LoweringContext*) const
	torch_xla::ir::LoweringContext::LowerNode(torch_xla::ir::Node const*)
	torch_xla::ir::LoweringContext::LoweringContext(std::string const&, torch_xla::Device, absl::lts_2020_02_25::Span<torch_xla::ir::Node const* const>, std::unordered_map<torch_xla::ir::Node const*, torch_xla::ir::Util::EmitStatus, std::hash<torch_xla::ir::Node const*>, std::equal_to<torch_xla::ir::Node const*>, std::allocator<std::pair<torch_xla::ir::Node const* const, torch_xla::ir::Util::EmitStatus> > >)
	torch_xla::XLATensor::Compile(std::vector<torch_xla::XLATensor, std::allocator<torch_xla::XLATensor> > const&, absl::lts_2020_02_25::Span<std::string const>, torch_xla::XLATensor::SyncTensorCollection const&, torch_xla::XLATensor::PostOrderData*)
	torch_xla::XLATensor::SyncTensorsGraphInternal(std::vector<torch_xla::XLATensor, std::allocator<torch_xla::XLATensor> >*, absl::lts_2020_02_25::Span<std::string const>, torch_xla::XLATensor::SyncTensorsConfig const&)
	torch_xla::XLATensor::SyncTensorsGraph(std::vector<torch_xla::XLATensor, std::allocator<torch_xla::XLATensor> >*, absl::lts_2020_02_25::Span<std::string const>, bool, bool)
	torch_xla::XLATensor::SyncLiveTensorsGraph(torch_xla::Device const*, absl::lts_2020_02_25::Span<std::string const>, bool)
	
	
	_PyMethodDef_RawFastCallKeywords
	_PyCFunction_FastCallKeywords
	_PyEval_EvalFrameDefault
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	_PyObject_FastCall_Prepend
	
	
	_PyEval_EvalFrameDefault
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	PyEval_EvalCode
	
	_PyMethodDef_RawFastCallKeywords
	_PyCFunction_FastCallKeywords
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	_PyObject_Call_Prepend
	PyObject_Call
	_PyEval_EvalFrameDefault
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	_PyFunction_FastCallKeywords
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	_PyObject_Call_Prepend
	PyObject_Call
	_PyEval_EvalFrameDefault
	_PyEval_EvalCodeWithName
	_PyFunction_FastCallKeywords
	_PyEval_EvalFrameDefault
	_PyObject_Call_Prepend
	_PyObject_FastCallKeywords
	
	_PyMethodDef_RawFastCallDict
	PyCFunction_Call
	_PyEval_EvalFrameDefault
	_PyFunction_FastCallKeywords
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	_PyFunction_FastCallKeywords
	_PyEval_EvalFrameDefault
	_PyFunction_FastCallKeywords
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	_PyEval_EvalCodeWithName
	_PyFunction_FastCallKeywords
	_PyEval_EvalFrameDefault
	_PyEval_EvalCodeWithName
	PyEval_EvalCode
	
	_PyMethodDef_RawFastCallKeywords
	_PyCFunction_FastCallKeywords
	_PyEval_EvalFrameDefault
	_PyEval_EvalCodeWithName
	_PyFunction_FastCallKeywords
	_PyEval_EvalFrameDefault
	_PyEval_EvalCodeWithName
	_PyFunction_FastCallDict
	
	
	_Py_UnixMain
	__libc_start_main
	_start
*** End stack trace ***
Scalar type not supported
Python Frames:
```

Similarly, once I got the following error:
```
RuntimeError: torch_xla/csrc/tensor_methods.cpp:880 : Check failed: xla::ShapeUtil::Compatible(shapes.back(), tensor_shape) 
```

## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->
Model training should have started but instead got the error.


Comments:
We didn't check yet whether BigBird works on TPU. We should put it on the roadmap (cc @vasudevgupta7) .

It should be interesting to check which operation (in bigbird) is causing problem on TPU :)

@vasudevgupta7 @patrickvonplaten Thanks. Please let us know if there was any update. Thanks!

Hi @mabdullah1994, sorry I missed your comment. I was checking bigbird on colab-tpu. I found that bigbird is working on TPU when we are not passing `attention_mask` (& only passing input_ids) into `model.forward()`. I will try to have a deeper look at it & will try to fix it some time soon.

Checkout this [notebook](https://colab.research.google.com/github/vasudevgupta7/bigbird/blob/main/notebooks/bigbird_tpu.ipynb) with TPU runtime.

Hi @vasudevgupta7 . Thanks for the update. Please let us know when this is fixed. Need this kind of urgently. Thanks!

@patrickvonplaten @vasudevgupta7 Any expected time frame, where we might expect it to work with `trainer` on TPUs? I am having the exact same problem, reproduction on synthetic dataset here on [Colab](https://colab.research.google.com/drive/1I6DR07ppQBTYBLatvGjBj70xsgVCOd4z?usp=sharing).

Or can we only use your script for the time being?