Adding burn::nn::Sigmoid

[RuelYasa]

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

#2030

Changes

Add a Sigmoid struct in nn just like Relu and Gelu etc.
Specifically:

1. Added sigmoid.rs in burn_core/src/nn
2. Added mod sigmoid; and pub use sigmoid::*; in burn_core/src/nn/mod.rs

Testing

1. cargo check succeeded.
2. mod tests succeeded.
3. run-checks all script failed due to

error: struct `ThreadId` is never constructed
  --> crates/burn-common/src/stub.rs:99:12
   |
99 | pub struct ThreadId(core::num::NonZeroU64);
   |            ^^^^^^^^
   |

, but it is seemingly an existing problem, which is irrelative to my modification.
An earlier PR also mentioned the problem.

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 84.25%. Comparing base (ed8a91d) to head (77aad0a).

Files	Patch %	Lines
crates/burn-core/src/nn/sigmoid.rs	75.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2031   +/-   ##
=======================================
  Coverage   84.25%   84.25%           
=======================================
  Files         845      846    +1     
  Lines      105444   105456   +12     
=======================================
+ Hits        88842    88853   +11     
- Misses      16602    16603    +1     

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[laggui]

Thanks for resolving your own issue 😄

LGTM! Just a minor change regarding the docstring.

[RuelYasa]

Oops... I forgot to git push!
It should be alright now.

[laggui]

Thanks!