SAM Metal error when using --generate-masks

I've tried to run the SAM example on Metal, using `--generate-masks`:

```
candle % cargo run --features metal --example segment-anything --release -- \
--image candle-examples/examples/yolo-v8/assets/bike.jpg \
--generate-masks
```

Gives error

```
Caused by:
Invalid matmul arguments [1792, 1] [1, 256] (64, 256, 256)
0: std::backtrace::Backtrace::create
1: std::backtrace::Backtrace::capture
2: candle_metal_kernels::err::MetalKernelError::bt
3: candle_metal_kernels::kernels::mlx_gemm::call_mlx_gemm
4: ::matmul
5: candle_core::storage::Storage::matmul
6: candle_core::tensor::Tensor::matmul
7: ::forward
8: ::forward
9: candle_transformers::models::segment_anything::mask_decoder::MaskDecoder::forward
10: candle_transformers::models::segment_anything::sam::Sam::generate_masks
11: segment_anything::main
12: std::sys::backtrace::__rust_begin_short_backtrace
13: std::rt::lang_start::{{closure}}
14: std::rt::lang_start_internal
15: _main
```

The example works without --generate-masks, but I can see the code paths are quite different.