feat: add AdaptiveAvgPool3d module (#4832)

[jaweed3]

Implements the 3D adaptive average pooling operation, closing #4832.

Added

• AdaptiveAvgPool3d module in burn-nn
• tensor::module::adaptive_avg_pool3d public API
• ModuleOps::adaptive_avg_pool3d + adaptive_avg_pool3d_backward trait methods
• IR structs, enum variants, builder macros, and visitor impls
• Backend implementations: ndarray (native Array5), cubecl (CPU fallback), tch (native), candle (panic), flex (all 4 dtypes), autodiff (backward with checkpointing), fusion, router, dispatch
• Forward tests (3) and autodiff backward tests (4)

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 272 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.91%. Comparing base (724bbb1) to head (718a208).
⚠️ Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/burn-cubecl/src/kernel/pool/base.rs	0.00%	110 Missing ⚠️
crates/burn-fusion/src/ops/module.rs	0.00%	40 Missing ⚠️
crates/burn-router/src/ops/module.rs	0.00%	25 Missing ⚠️
crates/burn-ir/src/operation.rs	0.00%	21 Missing ⚠️
crates/burn-router/src/interpreter.rs	0.00%	13 Missing ⚠️
crates/burn-fusion/src/stream/context.rs	0.00%	12 Missing ⚠️
crates/burn-cubecl/src/ops/module.rs	0.00%	9 Missing ⚠️
crates/burn-tch/src/ops/module.rs	0.00%	8 Missing ⚠️
crates/burn-autodiff/src/ops/module.rs	76.66%	7 Missing ⚠️
crates/burn-candle/src/ops/module.rs	0.00%	7 Missing ⚠️
... and 3 more

❌ Your patch check has failed because the patch coverage (60.00%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project check has failed because the head coverage (65.91%) is below the target coverage (80.00%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5115      +/-   ##
==========================================
+ Coverage   65.74%   65.91%   +0.16%     
==========================================
  Files        1182     1197      +15     
  Lines      179889   182832    +2943     
==========================================
+ Hits       118275   120520    +2245     
- Misses      61614    62312     +698     

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