Do not restore constant_buffer, and use bytes instead of Buffer

[lucylq]

Stack from ghstack (oldest at bottom):

• -> #15802
• #15801
• #15800

All constants are serialized in the segment (none in the Program).

This PR:

1. Places constant data into the PTEFile class instead of restoring it into the Program.
2. Use List[bytes] instead of List[Buffer] for constant and mutable data. Buffer was initially used to maintain alignment; now, constants are serialized with alignment in the segment, and Buffer is not required.

Update tests.

After this, we can mark 'constant_buffer' as deprecated, as it's no longer being used in deserialization or emitter.

Differential Revision: D86913756

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:x: 15 New Failures, 1 Unrelated Failure

As of commit dce4f45ab78eed8a5ebf440187c945ebe424d3ee with merge base 7600df87abadd0d486114c58b0c993e1aa70c8c5 ():

NEW FAILURES - The following jobs have failed:

• Lint / lintrunner / linux-job (gh)
• Lint / lintrunner-mypy / linux-job (gh) >>> Lint for backends/arm/test/misc/test_call_operator_submodule.py:
• pull / test-moshi-linux / linux-job (gh) RuntimeError: Command docker exec -t b7455376f633a30dc53460f95691bdccedabda4ceb43da46cde4a9d75ab26343 /exec failed with exit code 1
• pull / unittest / linux / linux-job (gh) backends/xnnpack/test/test_xnnpack_partitioner.py::TestXnnpackPartitioner::test_multi_method_partitioning_with_shared_weights
• pull / unittest / macos / macos-job (gh) backends/xnnpack/test/test_xnnpack_partitioner.py::TestXnnpackPartitioner::test_multi_method_partitioning_with_shared_weights
• pull / unittest-editable / linux / linux-job (gh) backends/xnnpack/test/test_xnnpack_partitioner.py::TestXnnpackPartitioner::test_multi_method_partitioning_with_shared_weights
• pull / unittest-editable / macos / macos-job (gh) backends/xnnpack/test/test_xnnpack_partitioner.py::TestXnnpackPartitioner::test_multi_method_partitioning_with_shared_weights
• Test CUDA Builds / test-model-cuda-e2e (google, gemma-3-4b-it, non-quantized) / linux-job (gh) RuntimeError: Command docker exec -t 470814691f045d590ec838035d86217d2d8baa89eee86f7373d617bcc4e25c69 /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (google, gemma-3-4b-it, quantized-int4-tile-packed) / linux-job (gh) RuntimeError: Command docker exec -t c6038c7cffa83b542ad9a4b61764b20f1539f55bfd8b58a26111f48ba78b1285 /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-large-v3-turbo, non-quantized) / linux-job (gh) RuntimeError: Command docker exec -t 92a7e08fd1ddfb5ec1283f9587f14e5a9f79fac8665b1b948ab6c19ea587fb3e /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-large-v3-turbo, quantized-int4-tile-packed) / linux-job (gh) RuntimeError: Command docker exec -t 8f553ffa32a14bd77acd5de0cdc037a8a06ab17d9695dc9454d086fb36405a87 /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-large-v3-turbo, quantized-int4-weight-only) / linux-job (gh) RuntimeError: Command docker exec -t 9fd6e23947c5e8e80d787c6946beb0a0301c7855ef0c6a130e263860e264c909 /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-small, non-quantized) / linux-job (gh) RuntimeError: Command docker exec -t 6cb6d13803c9a14efb9e88eee6036135700925b483936005cb4d75491c23cdcb /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-small, quantized-int4-tile-packed) / linux-job (gh) RuntimeError: Command docker exec -t af304d42d8ce418b5431d391311c5b3fc6e106cae76b2ee941ed4323abde4ba1 /exec failed with exit code 1
• Test CUDA Builds / test-model-cuda-e2e (openai, whisper-small, quantized-int4-weight-only) / linux-job (gh) RuntimeError: Command docker exec -t b6d313234fbd80ef946dc213b88c952f936a3e9d7313b24f736a596217e5aaad /exec failed with exit code 1

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

• Test Metal Backend / export-model-metal-artifact (openai, whisper-small, non-quantized) / macos-job (gh) (matched macos rule in flaky-rules.json) File doesn't exist

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