Merge branch 't5' of https://github.com/stas00/Megatron-LM into github-pr

The output will be two files named, in this case, `my-bert_text_sentence.bin` and `my-bert_text_sentence.idx`. The `--data-path` specified in later BERT training is the full path and new filename, but without the file extension.

For T5 use the same preprocessing as BERT, perhaps renaming it to:

<pre>

       --output-prefix my-t5 \

</pre>

Some minor modifications are required for GPT data preprocessing, namely, the addition of a merge table, an end-of-document token, removal of sentence splitting, and a change to the tokenizer type:

<pre>

python tools/preprocess_data.py \

         --micro-batch-size 16 \

         --global-batch-size 2048 \

         --vocab-file $VOCAB_FILE \

         --vocab-extra-ids 100 \

         --split 949,50,1 \

         --fp16"

OUTPUT_ARGS=&#60;same as those in <a href="#bert-pretraining">BERT pretraining</a> above&#62;

python pretrain_t5.py \

       $BERT_ARGS \

       $T5_ARGS \

       $OUTPUT_ARGS \

       --save $CHECKPOINT_PATH \

       --load $CHECKPOINT_PATH \