Update download.sh

[athul-22]

fix: Correct download.sh script for proper file handling and checksum validation

• Corrected file paths in wget commands to ensure files are downloaded to the correct locations.
• Adjusted sequence format for shard numbers to ensure zero padding.
• Ensured checksum validation works correctly for different CPU architectures (md5 for arm64 and md5sum for others).
• Added comments to explain changes and maintain clarity.

This update addresses the issue where the script prematurely closed and did not download specified models, ensuring proper functionality on Windows using bash with wget installed.

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