ggml : fix backward rope after YaRN (#3974)

* fix backward process of rope

rope backward process was broken after YaRN RoPE (#2268) implementation, due to missing changes in backward functions.

the code for the backward process is nearly identically to the forward process:
the only difference is the sign of the sin-values.

to avoid future regressions remove the near-duplicate backward functions and reuse the forward code:

for this a new function argument `bool forward` was added to `ggml_compute_forward_rope_f32` and `ggml_compute_forward_rope_f16`.
the sin-values will be negated when forward is false.

* fix finetune rope call to use correct default attn_factor of 1.0f

* remove unused `ggml_rope_xpos_back`

it is better to have only one `ggml_rope_back` function that accepts all rope parameters, so that `ggml_compute_backward` can propagate all parameters without having to switch between different rope_back variants.

* fix comments explaining the sinus sign in ggml_forward_rope

* add missing function arguments in declaration

* fix function argument type in declaration