bug: index_select() does not work on CUDA if index array has zero elements

[laptou]

Here's a minimal reproduction:

use candle_core::{Device, Tensor};

pub fn main() -> candle_core::Result<()> {
    let device = Device::Cpu;
    let indices = Tensor::from_vec::<_, u32>(vec![], 0, &device)?;
    let sample = Tensor::randn(0., 1., (3, 3, 3), &device)?;

    // works fine, produces a result with shape (0, 3, 3)
    let result = sample.index_select(&indices, 0)?;
    dbg!(result);

    let device = Device::new_cuda(0)?;
    let indices = Tensor::from_vec::<_, u32>(vec![], 0, &device)?;
    let sample = Tensor::randn(0., 1., (3, 3, 3), &device)?;

    // crashes with Cuda(Cuda(DriverError(CUDA_ERROR_INVALID_VALUE, "invalid argument")))
    let result = sample.index_select(&indices, 0)?;
    dbg!(result);

    Ok(())
}

candle-core version is 0.8.1