[ExecuTorch][Llama] Split custom sdpa op and kv cache

[kimishpatel]

Stack from ghstack (oldest at bottom):

• [ExecuTorch][BE] Split kv cache and SDPA for better code sharing #7413
• -> [ExecuTorch][Llama] Split custom sdpa op and kv cache #7412

Summary:
This enables us to do more easier module swap with model definitions
from torchtune

Test Plan:
CI

Reviewers:

Subscribers:

Tasks:

Tags:

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/7412

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❌ 2 New Failures

As of commit 7042327 with merge base 49cc399 ():

NEW FAILURES - The following jobs have failed:

• Check Labels / Check labels (gh)
  RuntimeError: Error checking labels: PR does not have required labels
• pull / unittest / macos / macos-job (gh)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d

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