fix: free ctx_copy in ggml_opt_free to plug per-training-session leak… #4

Job	Run time
android-ndk (arm64-cpu, -D ANDROID_ABI=arm64-v8a -D ANDROID_PLATFORM=android-31 -D CMAKE_TOOLCHAI...	7m 12s
android	13m 32s
android-ndk (arm64-snapdragon, --preset arm64-android-snapdragon-release)	16m 40s
	37m 24s

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