Custom RoPE + bettter memory management for CUDA (#2295)

* Custom RoPE + bettter memory management for CUDA

* Adjusted look ahead in ggml_cuda_pool_malloc to 5%

This is sufficient it seems.
We end up using about 200 MB less VRAM that way when running
the 13B model with context 8192.

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Co-authored-by: Iwan Kawrakow <iwan.kawrakow@gmail.com>