Cortex-M backend: Fix quantizer edge-cases

[AdrianLundell]

• Make sure not to annotate non-float tensors
• Handles the possibility of non-annotated nodes in the SharedQspecQuantizer

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

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❌ 4 New Failures

As of commit 36966d8 with merge base aed854f ():

NEW FAILURES - The following jobs have failed:

• pull / android / run-emulator (gh)
  The process '/opt/android/sdk/platform-tools/adb' failed with exit code 224
• trunk / test-arm-backend-ethos-u (test_pytest_models_ethos_u85) / linux-job (gh)
  RuntimeError: Command docker exec -t 11ad2c819712e60c2a73649e9b53aa417b488948d2830d652f48f9f8e9ea6d6c /exec failed with exit code 1
• trunk / test-mcu-cortex-m-backend / linux-job (gh)
  RuntimeError: Command docker exec -t a9f243c91f957a28de3fc85e175e6eda57c98c38c3f444661a2f1664ef41e1c8 /exec failed with exit code 1
• trunk / unittest-release / macos / macos-job (gh)
  backends/xnnpack/test/ops/test_conv2d.py::TestConv2d::test_fp16_conv2d

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[psiddh]

Nice fixes.

• Should we also add test case for Add support for quantized LeakyReLU #1 non-float input tensors.
  The CI for cortex m tests are broken because of my earlier changes and not this PR ( am looking into it)

[AdrianLundell]

Test fails unrelated