Add a method for substring(tokens / ids) checking to tokenizers.
# 🚀 Feature request

Adds a method to tokenizer to check whether a substring(tokens / ids) is included in evidence strings, or furthermore, get the starting and ending positions of the substring(tokens / ids).

The following code is an example for it:

```
from transformers import BertTokenizer

tokenizer = BertTokenizer.from_pretrained("bert-base-uncased")

input_str = ["There is a cute dog.", "There is a cute cat."]
substr = "cute dog"

input_ids = tokenizer(input_str).input_ids
sub_ids = tokenizer.encode(substr, add_special_tokens=False)

print(input_ids, sub_ids) # [[101, 2045, 2003, 1037, 10140, 3899, 1012, 102], [101, 2045, 2003, 1037, 10140, 4937, 1012, 102]] [10140, 3899]

# Added method
is_sub_tokens = tokenizer.is_sub_tokens(input_ids, sub_ids)

print(is_sub_tokens) # [True, False]
```

## Motivation

Some of question answering models require a label indicating whether an answer is included in the evidence strings, by adding this method, we can conveniently acquire this label.

## Your contribution

If no other concern and, I have some time, I'm willing to open a PR for it.

Any thoughts are welcomed :)
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