Nonzero should return an empty vec for zero tensors

[laggui]

Checklist

• Confirmed that run-checks all script has been executed.

Related Issues/PRs

Fixes #1946 #2180

Blocked by this wgpu fix in cubecl

Changes

Tensors where all elements are zero should return an empty vector since no valid indices represent non-zero elements.

Testing

Added unit test (currently fails with wgpu without the fix).

[codecov]

Codecov Report

Attention: Patch coverage is 84.61538% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.88%. Comparing base (96a2340) to head (7167674).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/burn-tch/src/ops/bool_tensor.rs	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2212   +/-   ##
=======================================
  Coverage   85.88%   85.88%           
=======================================
  Files         707      707           
  Lines       89805    89819   +14     
=======================================
+ Hits        77130    77143   +13     
- Misses      12675    12676    +1     

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[laggui]

Huh, I expected the added unit test to fail on wgpu without the cubecl fix merged 🤔 it fails locally when I test the same operation in a simple program

let tensor = Tensor::<Wgpu, 1, Bool>::from([false, false, false, false, false]);
let output = tensor.nonzero();

thread 'main' panicked at /home/laggui/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cubecl-runtime-0.2.0/src/memory_management/memory_pool/small.rs:109:9:
assertion `left == right` failed
  left: 0
 right: 32
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace