[ATen-vulkan] Enable deferred descriptor pool initialization (#121134)
Differential Revision: D54487619
## Context
Allow the descriptor pool of an `api::Context` object to be initialized in a deferred fashion, instead of forcing initialization upon construction. This mode of operation will be used in the ExecuTorch Vulkan delegate, where the exact number of descriptor sets can determined once the graph is built instead of needing to "guess" an adequate amount.
## Implementation Details
* Check `config.descriptorPoolMaxSets > 0` to check if the descriptor pool should be initialized
* Introduce `DescriptorPool::init()` function to trigger intialization
* Introduce safeguards against using an uninitialized descriptor pool
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121134
Approved by: https://github.com/manuelcandales
