reject variance of an empty tensor

[Boulea7]

What this changes

Tensor::variance() now panics with variance() of an empty tensor before reading the data when the tensor has no elements. Non-empty variance calculation is unchanged.

Why

Empty inputs previously returned NaN. This follows the requested caller-error behavior in #47.

Fixes #47.

How it was verified

The two empty-input tests failed on unmodified master because no panic occurred, then passed with the guard. Singleton and population-variance controls passed before and after the change.

• cargo test --locked --offline passes (CPU tests and doctests)
• cargo clippy --locked --offline --all-targets -- -D warnings is clean
• cargo fmt --check passes

Run on Linux with Rust 1.99. GPU runtime, Rust 1.87, release mode, and other operating systems were not tested locally.

Checklist

• Public items have /// docs
• No backward rule, kernel, dependency, or public signature was changed.

[CanReader]

Thank you for your changes, assertion check is very important for error handling and I forgot to add it while writing function! I'll check other functions. Thank you for your contributions.