Debug LukeForMaskedLM
# What does this PR do?
Fix some undesirable behaviors of `LukeForMaskedLM`.

#### 1. Make `LukeForMaskedLM` accept inputs without entity ids

**Before❌:**
```
>>> from transformers import LukeForMaskedLM, MLukeTokenizer
>>> import torch

>>> text = "test string"
>>> model = LukeForMaskedLM.from_pretrained('studio-ousia/luke-base')
>>> tokenizer = MLukeTokenizer.from_pretrained('studio-ousia/luke-base')
>>> encoding = tokenizer(text, return_tensors="pt")
>>> outputs = model(**encoding)
TypeError: linear(): argument 'input' (position 1) must be Tensor, not NoneType
```

I have fixed this by making entity inputs optional in the forward function.

#### 2. Make `LukeForMaskedLM` instantiable from `AutoModelForMaskedLM`

**Before❌:**
```
>>> from transformers import AutoModelForMaskedLM
>>> model = AutoModelForMaskedLM.from_pretrained("studio-ousia/luke-base")
ValueError: Unrecognized configuration class <class 'transformers.models.luke.configuration_luke.LukeConfig'> for this kind of AutoModel: AutoModelForMaskedLM.
```

I have fixed this by adding `("luke", "LukeForMaskedLM")` to `MODEL_FOR_MASKED_LM_MAPPING_NAMES`.

## Who can review?
@NielsRogge, could you check this PR? Thanks!
_The documentation is not available anymore as the PR was closed or merged._