Make code compatible with newest transformers version

[stefan-schroedl]

In https://github.com/georgian-io/Multimodal-Toolkit/blob/master/multimodal_transformers/model/tabular_transformers.py#L10, the statements
from transformers.modeling_bert import BERT_INPUTS_DOCSTRING (and others)
lead to an error with the most recent transformers package, and should be changed to:
from transformers.models.bert.modeling_bert import BERT_INPUTS_DOCSTRING etc