Update install_requirements.py to include TORCH_NIGHTLY_URL for requirement-examples.txt

[mergennachin]

For failing mosh test, https://hud.pytorch.org/hud/pytorch/executorch/main/1?per_page=50&name_filter=test-moshi-linux&mergeEphemeralLF=true

First, ./install_executorch.sh correctly installs PyTorch nightly:
Successfully installed torch-2.9.0.dev20250725+cpu

Then processes requirements-examples.txt which contains timm==1.0.7
When installing timm, pip detects a dependency conflict and downgrades PyTorch to 2.4.1

Let's try adding --extra-index-url

[pytorch-bot]

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❌ 2 New Failures, 3 Pending

As of commit 40d353e with merge base 4ce7078 ():

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
• pull / android / run-emulator (gh)
  The process '/usr/bin/sh' failed with exit code 255

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[kirklandsign]

  ERROR: HTTP error 403 while getting https://download.pytorch.org/whl/nightly/nvidia_cuda_nvrtc_cu12-12.1.105-py3-none-manylinux1_x86_64.whl (from https://download.pytorch.org/whl/nightly/cpu/nvidia-cuda-nvrtc-cu12/)
ERROR: Could not install requirement nvidia-cuda-nvrtc-cu12==12.1.105 from https://download.pytorch.org/whl/nightly/nvidia_cuda_nvrtc_cu12-12.1.105-py3-none-manylinux1_x86_64.whl (from torch->timm==1.0.7->-r requirements-examples.txt (line 4)) because of HTTP error 403 Client Error: Forbidden for url: https://download.pytorch.org/whl/nightly/nvidia_cuda_nvrtc_cu12-12.1.105-py3-none-manylinux1_x86_64.whl for URL https://download.pytorch.org/whl/nightly/nvidia_cuda_nvrtc_cu12-12.1.105-py3-none-manylinux1_x86_64.whl (from https://download.pytorch.org/whl/nightly/cpu/nvidia-cuda-nvrtc-cu12/)

Not sure where is that URL https://download.pytorch.org/whl/nightly/nvidia_cuda_nvrtc_cu12-12.1.105-py3-none-manylinux1_x86_64.whl from. Should be https://download.pytorch.org/whl/cu121/nvidia_cuda_nvrtc_cu12-12.1.105-py3-none-manylinux1_x86_64.whl#sha256=339b385f50c309763ca65456ec75e17bbefcbbf2893f462cb8b90584cd27a1c2 ?