rename pure-boilerplate fbcode-only TARGETS to BUCK

Summary:
Chunk 3 of fbcode/executorch TARGETS->BUCK migration. 8 directories where
only a TARGETS file existed (no sister BUCK). Each TARGETS is a no-arg
`define_common_targets()` call, and each shared `targets.bzl` was
verified to be xplat-compatible (no fbcode_macros loads, no fbcode-only
kwargs like keep_gpu_sections or re_test_utils).

Renaming TARGETS -> BUCK exposes these directories to xplat via dirsync.
Because the targets.bzl is xplat-compatible, the rules will materialize
in xplat for the first time — that is a deliberate side effect of the
migration to a single canonical build file per directory.

Directories migrated:
  - backends/aoti
  - backends/aoti/slim/c10/core
  - backends/aoti/slim/util
  - backends/aoti/slim/util/test
  - backends/qualcomm/partition
  - devtools/etdump/data_sinks/tests
  - examples/models/llama/experimental
  - extension/kernel_util/test

extension/training/examples/CIFAR was excluded after a trial run —
its targets.bzl uses a runtime.python_binary srcs attribute that the
xplat python_binary macro rejects ("default_only is not allowed to be
specified, but got [data_utils.py]"). It needs a manual fix to the
targets.bzl before it can be migrated.

Differential Revision: D109082059

Author: bigfootjon