feat: implement GPT training loop with multi-GPU and memory optimizations

- Add advanced memory footprint optimization using forward-activation recomputation for LayerNorm and GeLU.
- Optimize layer-wise activation buffer layout using a centralized `TensorSpec` registry to support large batch scaling.
- Integrate cuBLASLt matmul fusions, optional cuDNN attention layers, and stochastic rounding options.
- Fall back gracefully to `cudaMallocManaged` under heavy loads to prevent Outlier/OOM crashes.

Author: Eamon2009