#1869 Fix null reference errors when training from scratch with CUDA (#1907)
* #1869 Fix null reference errors when training from scratch with CUDA build
Calling ggml_compute_forward when node->src0 was null was causing train-text-from-scratch.exe to terminate unexpectedly.
* ggml : do not dereference src0 if NULL
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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>