move all ObjC (via metal-cpp) interaction until post static initializers (#370)

* move all ObjC (via metal-cpp) interaction until post static initializers

- metal-cpp relies on static initializers to cache class and selector pointers
- code in mlx was using metal-cpp to set up NSAutoreleasePools during its own static init time
- but this code was silently failing as the class and selector pointers from metal-cpp were still nil

- defer the creation of NSAutoreleasePools until after static init time
- ensure that we have coverage where autorelease pools are needed

* Update device.cpp

remove commented code

* Update device.cpp

remove commented out code

* Update scheduler.h

update comment

* per discussion use the pool inside the task() -- this will be metal only, not needed for cpu

* Update allocator.cpp

move pool to release/alloc area

mlx/backend/metal/allocator.cpp

@@ -29,6 +29,7 @@ BufferCache::BufferCache(MTL::Device* device)
     : device_(device), head_(nullptr), tail_(nullptr), pool_size_(0) {}
 
 BufferCache::~BufferCache() {
+  auto thread_pool = metal::new_scoped_memory_pool();
   clear();
 }
 
@@ -166,6 +167,8 @@ Buffer MetalAllocator::malloc(size_t size, bool allow_swap /* = false */) {
       return Buffer{nullptr};
     }
 
+    auto thread_pool = metal::new_scoped_memory_pool();
+
     // If we have a lot of memory pressure, check if we can reclaim some memory
     // from the cache
     if (device_->currentAllocatedSize() + size >= gc_limit_) {