Automatic FairMOT baseline model download and MOT17 download patch

[abuendia]

Description

This PR adds:

1. Automatic FairMOT model download in 01 and 02 tracking notebooks. The baseline dla model is uploaded to the public cvbp blob container and gets pulled down into both notebooks. Removes the need for the user to manually download the model from a Google Drive link.
2. Automatic download of MOT17 into 02 notebook. The relevant train/test subsets of MOT17 (2 GB) are uploaded to the public cvbp blob container and get pulled into the 02 notebook. I extract only the subdirectories needed for 02 to avoid a 5 GB download.
3. Minor changes to 01 and 02 markdown, i.e. typo/grammar fixes.

Reuses unzip_url util. Downloads if does not exist, and avoids re-downloading if exists. Adds relevant download URLs to data.py.

Important: When running the current version of 02 in staging, I encountered a bug that indicates the MOT17 zip from the MOT Challenge website was recently changed and corrupted. I have uploaded an earlier working version from @karineip to blob store.

Checklist:

• [x] I have followed the contribution guidelines and code style for this project.
• [x] I have added tests covering my contributions.
• [x] I have updated the documentation accordingly.
• [x] This PR is being made to staging and not master
• [x] I will squash merge this PR into staging

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