count the data_sources

[branfosj]

fix for #4474 - 100.0 files to download.

If count_files() returns 0 then the Rich progress bar defaults to percentage - so 100.0 for the total.

Downloading Base_epoch8_ckpt.pt_v12fc204edeae5b57713c5ad7dcb97d39 ━━━━━━━━━━━━━━━━━━━━━━━━╺━━━━━━━━━━━━━━━ 294.6/483.6 MB 7.6 MB/s 0:00:25
Fetching files:   7% (7/100.0) ━━╸━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 0:00:38 (Base_epoch8_ckpt.pt_v12fc204edeae5b57713c5ad7dcb97d39)

[jfgrimm]

lgtm