ValueError if answer in truncated table rows and columns in Tapas tokenization
Author: kolk
Pull request: no

## Environment info
<!-- You can run the command `transformers-cli env` and copy-and-paste its output below.
     Don't forget to fill out the missing fields in that output! -->

- `transformers` version: 4.17.0
- Platform: Ubuntu 20 LTS
- Python version: 3.8.10 
- PyTorch version (GPU?): 1.9.0
- Tensorflow version (GPU?): 2.8.0
- Using GPU in script?: No
- Using distributed or parallel set-up in script?: No

### Who can help
TAPAS: @NielsRogge

## Information

Model I am using (TAPAS):

The problem arises when using:
* [x] the official example scripts: (give details below)
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [x] my own task or dataset: (give details below)

## Description
Tapas tokenization truncates long tables to `num_rows`  when truncation strategy is `drop_rows_to_fit`. It does not truncate the dropped rows and columns from `answer_coordinates`.  If `answer_coordinates` contains the dropped rows, it results in `ValueError: Couldn't find all answers`  in `_get_answer_ids` due to mismatch in `row_ids`, `col_ids` and `answer_coordinates`
## To reproduce

Steps to reproduce the behavior:


1. Initialize a large table, and required fields. Answer coordinates exceeds the `model_max_length`
```python
import numpy as np
import pandas as pd
from transformers import AutoTokenizer
tokenizer = AutoTokenizer.from_pretrained("google/tapas-base-finetuned-wikisql-supervised")
tab = np.random.choice(5, 513)
table = pd.DataFrame(data=tab, columns=["Value"]).astype('str')
answer_texts = [table.iloc[512]["Value"]]
answer_coordinates=[(512,0)]
question="dummy question"
```
2.  Tokenize the large table
```python
encoding = tokenizer(
    table=table,
    queries=question,
    answer_coordinates=answer_coordinates,
    answer_text=answer_texts,
    truncation=True,
    padding="max_length",
    return_tensors="pt",
)
```
Output:
```python
Traceback (most recent call last):
  File "/py3/lib/python3.8/site-packages/IPython/core/interactiveshell.py", line 3441, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-13-2827f7bd32ad>", line 1, in <module>
    encoding = tokenizer(
  File "/py3lib/python3.8/site-packages/transformers/models/tapas/tokenization_tapas.py", line 624, in __call__
    return self.encode_plus(
  File "/py3/lib/python3.8/site-packages/transformers/models/tapas/tokenization_tapas.py", line 990, in encode_plus
    return self._encode_plus(
  File "/py3/lib/python3.8/site-packages/transformers/models/tapas/tokenization_tapas.py", line 1044, in _encode_plus
    return self.prepare_for_model(
  File "//py3/lib/python3.8/site-packages/transformers/models/tapas/tokenization_tapas.py", line 1203, in prepare_for_model
    labels = self.get_answer_ids(column_ids, row_ids, table_data, answer_text, answer_coordinates)
  File "/py3/lib/python3.8/site-packages/transformers/models/tapas/tokenization_tapas.py", line 1789, in get_answer_ids
    return self._get_answer_ids(column_ids, row_ids, answer_coordinates_question)
  File /py3/lib/python3.8/site-packages/transformers/models/tapas/tokenization_tapas.py", line 1778, in _get_answer_ids
    raise ValueError("Couldn't find all answers")
ValueError: Couldn't find all answers
```
## Expected behavior
Remove truncated rows and columns from `answer_coordinates` and return the truncated `labels`. Throw an exception if `answer_coordinates` is empty.


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