Pull Request resolved: #13474



* Allocate memory for weight tensors right before the prepacking shader is dispatched, rather than while building the graph
* Move allocation of shared objects (i.e. memory for intermediate tensors) to occur after prepacking

## Motivation

Prevent screen blackout (Llama 3.2 1B) / device crash (Llama 3.2 3B) when running Llama 3.2 models on Samsung Galaxy S24. This behaviour is related to high peak memory usage when loading the model.

## Full Context

During model loading, Vulkan delegate needs to store 3 copies of constant data in memory at various points:

* source data obtained from loading the model
* staging buffer
* GPU texture/buffer

The general rationale of this change is to allocate memory for each copy only when necessary to minimize the "overlap" when all 3 exist at once.

### Current Order of operations

Legend:
* `W` represents total weight nbytes
* `w` represents weight nbytes for one tensor
* `A` represents total activations nbytes
* `M` represents approximation of total memory footprint

First, model file is loaded

Then, when building compute graph, for each weight tensor:
1. Weight data is loaded from NamedDataMap (`M = W`)
2. GPU texture/buffer for weight is initialized + memory allocated (`M = 2W`)
3. After building the graph, `graph->prepare()` is called which currently allocates memory for the activation tensors as well (`M = 2W + A`)

Then, during the prepacking stage for each weight tensor, each weight tensor is copied individually:
1. Staging buffer initialized (`M = 2W + A + w`)
2. Copy CPU weight data to staging + CPU Weight data is freed (`M = 2W + A`)
3. Compute shader dispatch to copy staging to GPU texture/buffer + free staging buffer (`M = 2W + A - w`)

The peak usage in mainline will be `M = 2W + A + w`

### Revised order of operations

This change revises the order of operations:

1. Weight data is loaded from NamedDataMap (`M = W`)
2. GPU texture/buffer for weight is initialized, but **memory is not allocated** (`M = W`)

Then, during the prepacking stage for each weight tensor, each weight tensor is copied individually:
1. Staging buffer initialized (`M = W + w`)
2. **Memory allocated for GPU texture/buffer** (`M = W + 2w`)
3. Copy CPU weight data to staging + CPU Weight data is freed (`M = W + w`)
4. Compute shader dispatch to copy staging to GPU texture/buffer + free staging buffer (`M = W`)

**Then, after all prepacking operations complete, only then is Activation memory allocated** (`M = W + A`)

Under this scheme, peak memory is reduced to `M = W + A` (or alternatively `M = W + 2w` if `2w > A`) which is (or at least very close to) the theoretical minimum.
ghstack-source-id: 303862303

Differential Revision: [D80460033](https://our.internmc.facebook.com/intern/diff/D80460033/)