[sharmis-tp]

Summary

• Add assert!(fan_in > 0) in kaiming_uniform, xavier_uniform, and uniform_bias so a zero fan panics with a message naming the shape instead of producing infinity and crashing inside rand
• Return vec![lo; n] from rng::uniform when lo == hi instead of panicking on the empty range
• Add #[should_panic] tests for Linear::new(0, 10), Linear::no_bias(0, 10), and Tensor::xavier_uniform with zero fan, plus a test that rng::uniform handles the degenerate range

Closes #48