[ET-VK][Llama] Apply XNNPACK partitoner as well when lowering to Vulkan (#6857)

* [ET-VK] Enforce GPU buffer limit when partitioning

Pull Request resolved: https://github.com/pytorch/executorch/pull/6829

## Context

In Vulkan, there is a limit on the number of elements a GPU buffer can have. If a GPU buffer exceeds this limit, then the API will either produce an error or undefined behaviour will ensue.

## Changes

Along with `texture_limits`, introduce a configurable `buffer_limit` entry in the partitioner configuration.
ghstack-source-id: 253568943

Differential Revision: [D65899828](https://our.internmc.facebook.com/intern/diff/D65899828/)

* [ET-VK][Llama] Apply XNNPACK partitoner as well when lowering to Vulkan

Pull Request resolved: https://github.com/pytorch/executorch/pull/6830

## Context

The final logit linear layer in the Transformer architecture has extremely large tensors, since the output and weight tensors will have a tensor with dim equal to the vocabulary size, which may be extremely large. Because of this, image textures cannot be used to execute the op when running with the Vulkan delegate, so an implementation using buffer based tensors must be used.

Unfortunately, Vulkan does not have a performant implementation of linear with buffer based tensors at the moment. As a result, if this final linear layer is executed in Vulkan, model inference is extremely slow.

## Changes

The below diff will prevent the final logit linear layer from being delegated to Vulkan by enforcing a GPU buffer limit.

This diff modifies the export llama script to apply the XNNPACK partitioner after the Vulkan partitioner if lowering to Vulkan, to ensure that remaining ops will be accelerated with XNNPACK. 4 bit quantization will also apply an additional Quantizer after applying the Vulkan quantizer (which will skip the final logit linear layer) so that the final logit linear can be quantized as well.

## Long Term

This is a temporary measure while an optimized buffer based linear implementation is developed. Once the Vulkan implementation achieves parity with XNNPACK, the final logit linear will be delegated to Vulkan once more.
ghstack-source-id: 253568942

Differential Revision: [D65899827](https://our.internmc.facebook.com/intern/diff/D65899827/)

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Co-authored-by: Stephen Jia <ssjia@meta.com>