setup script false positive TPU detection

[chronicle95]

As the title says, it detects that my system has TPU in it, where it most certainly does not.

An empty /sys/class/accel directory is present in the system, that's why the OR condition activates.

[ayaangazali]

I'd like to take this. The [ -d /sys/class/accel ] test in setup is true for an empty directory, so a machine with an empty /sys/class/accel gets misclassified as TPU and pulls jax[tpu]. Fix: require the directory to be non-empty. PR incoming.