Hey thanks for the input.
I found the mistake in the understanding 

INFO:./train_decoder_only.py:Train dataset length: 112449

Here we are printing the length of dataset:

len(train_dataset)

Where are

Epoch: [1/28113] 0%| [00:00<00:01]

is the length of the dataset loader with batch size 4.  so 1122449/4=28113 which is, unfortunately, the length of validation dataset  leading to all the confusion.