Arm backend: Move tosa files to tosa/ directory

[Sebastian-Larsson]

cc @digantdesai @freddan80 @per @zingo @oscarandersson8218

[pytorch-bot]

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❌ 2 New Failures, 2 Pending, 1 Unrelated Failure

As of commit 8cade3a with merge base 6aac82a ():

NEW FAILURES - The following jobs have failed:

• Build documentation / build (buck2) / Build doc (gh)
  At least one of the pre-conditions you specified did not hold
• trunk / unittest-release / macos / macos-job (gh)
  backends/xnnpack/test/ops/test_rsqrt.py::TestRsqrt::test_fp16_rsqrt

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• trunk / test-models-linux-aarch64 (llama3_2_vision_encoder, portable, linux.arm64.2xlarge) / linux-job (gh) (matched linux rule in flaky-rules.json)
  docker: Error response from daemon: unauthorized: authentication required.

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[zingo]

Fails are unrelated

[digantdesai]

will cause problems internally, let's see. I hope I can fwd fix.

[shoumikhin]

@zingo @digantdesai need to address

executorch/backends/arm/TARGETS

Lines 40 to 41 in b02db12

	"tosa_backend.py",
	"tosa_partitioner.py",

at least

[zingo]

Sorry we cant build or see the error but we tried to fix it with this #14006