"Write With Transformer" inserts a space whenever accepting a suggestion, even if a space doesn't belong there
# 🐛 Bug

## Information

Model I am using (Bert, XLNet ...): GPT-2

Language I am using the model on (English, Chinese ...): English

The problem arises when using:
* [X] the official example scripts: Write With Transformer
* [ ] my own modified scripts: (give details below)

The tasks I am working on is:
* [X] an official GLUE/SQUaD task: Write With Transformer
* [ ] my own task or dataset: (give details below)

## To reproduce

Steps to reproduce the behavior:

1. Type something ending in the middle of a word. (e.g. `See how a modern neural netw`) Alternatively, type something ending with an open parenthesis or quotation mark, such as `Donald Trump tweeted "`.
2. Press Tab and accept a suggestion.
3. Observe how a space is added prior to the accepted text, despite a space not belonging there.

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## Expected behavior

I'm not sure about the other models, but I know GPT-2 does start continuations with a space when appropriate. So it should be possible to have it only add a space when a space is desirable. Otherwise, I think it would be better to have it not add a space at all, as it's easier to manually add a space before pushing Tab (if one is desired) than it is to go back and delete undesired spaces every time they're generated.

## Environment info
N/A
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