[ATen-Vulkan][EZ] Small fixes: fix gpu size calculation and Half scalartype ctype mapping (#122096)
Summary:
## Context
Some small fixes to the ATen-Vulkan backend.
The first is that GPU sizes for a 4 dimensional tensor with width packing had a small bug:
```
case 4:
switch (memory_layout) {
case api::GPUMemoryLayout::TENSOR_WIDTH_PACKED:
gpu_sizes.at(0) = sizes.at(0);
gpu_sizes.at(1) = sizes.at(1);
// should be gpu_sizes.at(2) == sizes.at(2)
gpu_sizes.at(2) = sizes.at(3);
gpu_sizes.at(3) = api::utils::align_up(sizes.at(3), INT64_C(4));
break;
```
This was fixed by simplifying the logic of GPU size calculation for texture storage.
The second was to modify the ctype mapping of the `api::kHalf` scalar type to be `float` instead of `unsigned short`. This is because GLSL does not natively support `float16`, so even with a FP16 texture type CPU/GPU transfer shaders will have to read from and write to `float` buffers.
In the future, we will look into integrating [VK_KHR_shader_float16_int8](https://registry.khronos.org/vulkan/specs/1.3-extensions/man/html/VK_KHR_shader_float16_int8.html) into ATen-Vulkan to allow for 16 bit and 8 bit types to be referenced explicitly.
Test Plan: CI
Differential Revision: D55018171
Pull Request resolved: https://github.com/pytorch/pytorch/pull/122096
Approved by: https://github.com/jorgep31415
