Improve pickle (CandleTensor) conversions to NestedValue

[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 #1773

Changes

1. Made value attribute of TensorData public
2. Manually serialize tensor

Testing

1. All unit tests passing
2. Manually tested whisper model conversion.

Here are time improvements:

Old version:
cargo run --bin convert --release
0.98s user 0.11s system 89% cpu 1.217 total

cargo run --bin convert (DEBUG)
12.21s user 0.14s system 88% cpu 13.960 total

New version:
cargo run --bin convert --release
0.68s user 0.10s system 86% cpu 0.903 total

cargo run --bin convert (DEBUG)
9.19s user 0.14s system 85% cpu 10.943 total

44% improvement for the release mode.

[codecov]

Codecov Report

Attention: Patch coverage is 86.48649% with 5 lines in your changes missing coverage. Please review.

Project coverage is 85.05%. Comparing base (98a58c8) to head (b9d5f09).
Report is 6 commits behind head on main.

Files	Patch %	Lines
crates/burn-tensor/src/tensor/data.rs	72.22%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1944   +/-   ##
=======================================
  Coverage   85.04%   85.05%           
=======================================
  Files         793      793           
  Lines       94590    94601   +11     
=======================================
+ Hits        80442    80460   +18     
+ Misses      14148    14141    -7     

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[laggui]

🚀 🚀

Awesome, thanks for making the changes!