ggml : add ggml_row_size() (fixes llama out of space) (#4461)

* Fixes "Not enough space in the context's memory pool" encountered on certain models, which seems to be caused by some imprecision related to the automatic casting of floating point values

* do not cast to size_t, instead just use doubles

* ggml : add ggml_row_size(), deprecate ggml_type_sizef()

* ggml : fix row size compute to avoid overflows

* tests : fix sizey -> sizez

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Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>