Add support to DistilBertLMHeadModel
Author: kwang2049
Pull request: no

# 🚀 Feature request

## Background
Hi, I am the first author of the paper ["**TSDAE**: Using Transformer-based Sequential Denoising Auto-Encoder for Unsupervised Sentence Embedding Learning"](https://arxiv.org/abs/2104.06979). The work proposed a new method, TSDAE to train unsupervised-sentence-embedding models with denoising **autoencoder** loss on top of BERT-like PLMs. The code of TSDAE has been integrated into the SBERT repo and one can find the source file [here](https://github.com/UKPLab/sentence-transformers/blob/master/sentence_transformers/losses/DenoisingAutoEncoderLoss.py) and the example training script [here](https://github.com/UKPLab/sentence-transformers/tree/master/examples/unsupervised_learning/TSDAE).

## What is missing
As one can tell from the "autoencoder" in the name, TSDAE needs a decoder (i.e. `LMHead`) to work. However, the popular [**DistilBERT**](https://github.com/huggingface/transformers/blob/master/src/transformers/models/distilbert/modeling_distilbert.py) in the current HF-transformers **does not support it as a decoder** (i.e. missing something called `DistilBertLMHeadModel`). 

Previously, I **used to bypass** this issue with [my personal extension](https://gist.github.com/kwang2049/1f0e1f0ce119456284c0af048ba097a7) and one can find the [discussion/issues](https://github.com/UKPLab/sentence-transformers/issues/962) in the SBERT repo about this. Recently, as users reported, I have just found **this way is not really maintainable and continuable**: HF-transformers keeps updating the code all the time and I need to update the extension accordingly then.

## Goal
Add support to **DistilBertLMHeadModel**. Actually, **I have made the changes in my fork** (for PyTorch) and it just needs to be merged (of course, after the code review from the HF community).

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## Motivation
With support to DistilBertLMHeadModel, all the decoding functions will be possible for all the amazing distilbert models, e.g. the TSDAE learning, seq2seq tasks, etc.

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## Your contribution
I have forked the latest master branch and added the corresponding support for PyTorch. One can check the pull-request for details. Really appreciate it if this issue can be solved.

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