Re land asset management (#14768)

Summary:
This re-lands #13560

To see changes from first attempt, see 16ff41b...428c1c4

Several notable changes:

1. In the original PR, we removed a backup path where if the transaction to store the compiled asset fails, the prediction could still succeed (https://github.com/pytorch/executorch/pull/13560/files#r2289523798).  This is restored here: 16ff41b...428c1c4#diff-ce24872f2e45ee7bf53f50b319b8f77f2f340b01b0889748deaaa754084d4fefR495-R509

2. We add new logic to re-create the asset directory if needed inside _storeAssetAtURL: 16ff41b...428c1c4#diff-4178f5b6716f52c9cb12ee99be51a9d69d3ed0e4609e9afb0429571cf9cf84a3R468

3. We add new logic to re-create the trash directory if needed inside moveItemAtURLToTrash (16ff41b...428c1c4#diff-4178f5b6716f52c9cb12ee99be51a9d69d3ed0e4609e9afb0429571cf9cf84a3R432).  Previously this function was called move_to_directory.

4. The previous code had a nested transaction inside _storeAssetAtURL (because _removeAssetAtURL also starts a transaction).  We remove the nested transaction by passing in alreadyInsideTransaction to _removeAssetAtURL).

5. We add new NSFileProtectionCompleteUntilFirstUserAuthentication to our directories and database files.

In addition to the above changes, we cleaned up some code:

1. The function create_directory_if_needed is removed from backend_delegate.mm: 16ff41b...428c1c4#diff-89543b8e2fe49bec8243d18801819bc2594e86d60af8823c03ac2c3e6fc7607dL48.  An equivalent function exists in the asset manager.

2. Introduced loadModelWithCompiledAsset and use it rather than using loadModelWithContentsOfURL (16ff41b...428c1c4#diff-ce24872f2e45ee7bf53f50b319b8f77f2f340b01b0889748deaaa754084d4fefL585)


Reviewed By: mergennachin

Differential Revision: D83893376

Pulled By: metascroy

Author: metascroy

backends/apple/coreml/runtime/delegate/ETCoreMLAssetManager.h

@@ -99,6 +99,17 @@ NS_ASSUME_NONNULL_BEGIN
 - (NSUInteger)compact:(NSUInteger)sizeInBytes error:(NSError* __autoreleasing*)error;
 
 
+/// Executes a block with a unique temporary directory.
+///
+/// A new temporary subdirectory URL is created inside the receiver’s designated
+/// base directory. The directory is passed to the block, which can use it to
+/// perform temporary file operations. After the block finishes executing,
+/// the directory and its contents are removed.
+///
+/// @param block A block to execute. The block receives a unique URL.
+- (void)withTemporaryDirectory:(void (^)(NSURL* directoryURL))block;
+
+
 /// Purges the assets storage. The assets are moved to the trash directory and are asynchronously
 /// deleted.
 ///
@@ -117,6 +128,12 @@ NS_ASSUME_NONNULL_BEGIN
 /// contents are deleted asynchronously.
 @property (copy, readonly, nonatomic) NSURL* trashDirectoryURL;
 
+
+/// The staging directory URL, used to hold assets that are being prepared or processed
+/// before they are moved into their final location. The contents of this directory
+/// are temporary and may be cleared when no longer needed.
+@property (copy, readonly, nonatomic) NSURL* stagingDirectoryURL;
+
 /// The file manager.
 @property (strong, readonly, nonatomic) NSFileManager* fileManager;