Update version to 0.21.0

[laggui]

Checklist

• Confirmed that cargo run-checks command has been executed.
• Made sure the book is up to date with changes in this PR.

Changes

• Update version to 0.21.0 for major release
• Update cubecl and cubek published versions
• Fix mnist model binary
• Add topk default implementation for backends & override tch implementation

Testing

Unit tests

[codecov]

Codecov Report

❌ Patch coverage is 69.81132% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.35%. Comparing base (7640aa7) to head (a2f5dd2).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/burn-tch/src/ops/base.rs	0.00%	8 Missing ⚠️
crates/burn-tch/src/ops/tensor.rs	0.00%	6 Missing ⚠️
crates/burn-tch/src/ops/int_tensor.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4927      +/-   ##
==========================================
+ Coverage   65.29%   65.35%   +0.06%     
==========================================
  Files        1170     1170              
  Lines      174856   175018     +162     
==========================================
+ Hits       114169   114381     +212     
+ Misses      60687    60637      -50     

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