ggml : collect all input tensors into graph_inputs (#29634)

graph_inputs was populated while splitting the graph, so it only
contained the inputs that are used as srcs of some node. With pipeline
parallelism (n_copies > 1), each graph input contributes n_copies leafs
to graph_copy, so switching between batches that consume different
inputs (e.g. token batches that do not use the embeddings input vs
image batches that do) changed the graph composition. This shifted the
input copies in graph_copy, making the backend ids comparison report
spurious changes and forcing the scheduler to re-reserve. The
re-reserve could then record smaller input sizes (e.g. out_ids with
n_outputs = 0) and abort later on a graph with an unchanged size via
GGML_SCHED_DEBUG_REALLOC.

Collect the inputs after the split instead, from all input leafs of the
graph, so that the graph composition depends only on which inputs
exist, not on which inputs are used.

Assisted-by: pi:llama.cpp/MiMo-V2.6-Flash-RL