Fix libtpu version for torch and do not pre-install tensorflow-tpu on TPU.

[metrizable]

We install a libtpu version compatible with both jax 0.7.2 and torch 2.8.0. Why? tunix latest -> flax 0.12 -> jax 0.7.2 -> libtpu 0.0.23, and that libtpu version causes pjrt api errors for torch 2.8.0:

pjrt_c_api_helpers.cc:258] Unexpected error status Unexpected PJRT_Plugin_Attributes_Args size: expe cted 32, got 24. The plugin is likely built with a later version than the framework. This plugin is built with PJRT API version 0.75. 

• https://github.com/pytorch/xla/blob/d517649bdef6ab0519c30c704bde8779c8216502/setup.py#L111
• https://github.com/jax-ml/jax/blob/3489529b38d1f11d1e5caf4540775aadd5f2cdda/setup.py#L26

Of particular note, we no longer pre-install tensorflow-tpu as the newer libtpu causes issues finding the TPUs

external/local_xla/xla/stream_executor/tpu/tpu_platform_interface.cc:78] No TPU platform found. Platform manager status: OK 

We also update how we install Python packages via uv for consistency and reproducibility. From a requirements.in file, we first generate a consistent dependency closure via uv pip compile, and then uv pip install the packages from the generated requirements.txt.

[calderjo]

awesome thanks!