Why LSTM only accept fixed length input

Course Q&A Natural Language Processing NLP with Sequence Models
1

The slides shows LSTM only accepts fixed length input

image
image903×523 73.2 KB

Why is that? I thought the program can take the variable length input to LSTM units and just stop when sentence ends.

Does variable length input introduce problems in cost function calculation or backward prop?

2

LSTM does accept variable sequence size as input. However, for each batch, the sequences need to be the same size.