The batch_size tensor is telling you that:

Your longest sequence is length 5 (because the tensor has five entries)
Three of those entries contain data at the 0th position.
Two of those entries contain data at the 1st position
One of those entries contains data at the 2nd position
One of those entries contains data at the 3rd position
One of those entries contains data at the 4th position

Those are all true statements:

Your longest tensor is a, consisting of five 0’s.  (And note the presence and location of the five 0s in the data tensor.)
Your tensor c has only 1 element
Your tensor b has only 2 elements

Most importantly/intuitively:  Batch_sizes are not lengths.