Implement Pooler layer in BertModelLayer

[mrinaald]

Implement the Pooler layer from the BERT model architecture, which creates a pooled feature vector using the first token from the output sequence. In many of the online blogs and examples, they mention to take the pooled output from BERT directly and add dense layers (or other layers) on this pooled output.

With this change, the pooler layer weights available in the downloaded checkpoint files of various models can also be loaded into the BertModelLayer object.

• Original Behaviour:
  Done loading 37 BERT weights from: ~/Downloads/BERT/BERT-Weights/uncased_L-2_H-128_A-2/bert_model.ckpt into <bert.model.BertModelLayer object at 0x7f6a9c64df40> (prefix:bert_orig). Count of weights not found in the checkpoint was: [0]. Count of weights with mismatched shape: [0]
  Unused weights from checkpoint:
  bert/pooler/dense/bias
  bert/pooler/dense/kernel
  cls/predictions/output_bias
  cls/predictions/transform/LayerNorm/beta
  cls/predictions/transform/LayerNorm/gamma
  cls/predictions/transform/dense/bias
  cls/predictions/transform/dense/kernel
  cls/seq_relationship/output_bias
  cls/seq_relationship/output_weights

• Modified Behaviour:
  Done loading 39 BERT weights from: ~/Downloads/BERT/BERT-Weights/uncased_L-2_H-128_A-2/bert_model.ckpt into <bert.model.BertModelLayer object at 0x7f6a9d026a30> (prefix:bert_pooled). Count of weights not found in the checkpoint was: [0]. Count of weights with mismatched shape: [0]
  Unused weights from checkpoint:
  cls/predictions/output_bias
  cls/predictions/transform/LayerNorm/beta
  cls/predictions/transform/LayerNorm/gamma
  cls/predictions/transform/dense/bias
  cls/predictions/transform/dense/kernel
  cls/seq_relationship/output_bias
  cls/seq_relationship/output_weights

To get the pooler layer output, we need to initialize the BertModelLayer as follows:
bert_params.return_pooler_output = True
l_bert = bert.BertModelLayer.from_params(bert_params, name="bert")

[Ahmedn1]

Would someone merge this branch, please?