[ET-VK][EZ] Fix StorageBuffer size in PrepackNode

Fixing a bug I wrote, but in my defense the bug was in the code I copied. 😛

This change decreases the size of our staging `StorageBuffer`s which were previously bigger than necessary.

There's a few numbers describing size at play:
```
numel -> CPU buffer
nbytes -> numel * cpu_dtype
gpu_numel -> GPU texture
gpu_nbytes -> gpu_numel * gpu_dtype
```

Firstly, `StorageBuffer`'s ctor takes `numel` not `nbytes`. Secondly, we should use the CPU size not the GPU size which may be aligned up to a multiple of 4.

Differential Revision: [D55619076](https://our.internmc.facebook.com/intern/diff/D55619076/)

ghstack-source-id: 220995380
Pull Request resolved: #2811

Author: jorgep31415

backends/vulkan/runtime/graph/ops/PrepackNode.cpp

@@ -39,10 +39,8 @@ void PrepackNode::encode(ComputeGraph* graph) {
   TensorRef tref = graph->get_val(tref_).toTensorRef();
   vTensor packed = graph->get_val(packed_).toTensor();
 
-  // TODO: Extract to standalone function, to support other types of prepacking.
-  api::StorageBuffer staging(
-      graph->context(), packed.dtype(), packed.gpu_nbytes());
   size_t numel = api::utils::multiply_integers(tref.sizes);
+  api::StorageBuffer staging(graph->context(), tref.dtype, numel);
   size_t nbytes = numel * api::element_size(tref.dtype);
   copy_ptr_to_staging(tref.data, staging, nbytes);