Use of loader-specific image_filenames() in NetVLAD

[akshay-krishnan]

image_filenames used in NetVLAD retreiver (for visualization) is not present in all loaders (not a base class member). This will crash if used with a loader that does not have the method.

gtsfm/gtsfm/retriever/netvlad_retriever.py

Line 200 in 581f84a

query_names = loader.image_filenames()