Add sign tensor operator

[antimora]

Pull Request Template

Checklist

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

Related Issues/PRs

Fixes #522

Changes

1. Default sign OP in Numerical trait
2. Backwards autodiff
3. Ndarray and Tch backends
4. Docs are updated

The Candle backend does not support currently (see huggingface/candle#1827 and #1447) and will use default implementation. WGPU uses default implementation as well (#1445 is filed to improve).

Testing

1. New unit tests
2. run-checks succeeded.

[codecov]

Codecov Report

Attention: Patch coverage is 87.30159% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 85.76%. Comparing base (56f4602) to head (441e7a3).

Files	Patch %	Lines
crates/burn-tch/src/ops/int_tensor.rs	0.00%	5 Missing ⚠️
crates/burn-tch/src/ops/tensor.rs	0.00%	5 Missing ⚠️
crates/burn-autodiff/src/ops/int_tensor.rs	0.00%	3 Missing ⚠️
crates/burn-tch/src/ops/base.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #1446    +/-   ##
========================================
  Coverage   85.76%   85.76%            
========================================
  Files         640      642     +2     
  Lines       70632    70758   +126     
========================================
+ Hits        60577    60687   +110     
- Misses      10055    10071    +16     

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[louisfd]

LGTM