torchrec tutorial: old cuda version install

[seanv507]

https://github.com/pytorch/tutorials/blob/a3695df13baac449f92a06b1029260a21f2e5c90/intermediate_source/torchrec_intro_tutorial.py#L38C1-L43C3

the current lines try to install a cuda 12.1 version when the current colab is 12.6 (and so torch remains at 12.6)
changing the index to cu126 fails, because torchrec requires tensordict, which is not in the index?

my proposed solution is to remove the pytorch install line
and add tensordict to torchmetrics line

# !pip3 install --pre torch --index-url https://download.pytorch.org/whl/cu121 -U remove
!pip3 install fbgemm_gpu --index-url https://download.pytorch.org/whl/cu126
!pip3 install torchmetrics==1.0.3 tensordict
!pip3 install torchrec --index-url https://download.pytorch.org/whl/cu126

cc @PaulZhang12 @iamzainhuda @vmoens @nairbv