model : glm-dsa load DSA indexer tensors as optional (#24770)

GLM-5.2 ships the DSA "lightning indexer" on only a subset of layers (the "full" layers; others omit it), but the GLM_DSA loader created the five indexer tensors on every layer as required, so loading any GLM-5.2 GGUF failed with e.g. missing tensor 'blk.3.indexer.k_norm.weight'.

GLM_DSA's graph is llama_model_deepseek2::graph (plain MLA) and does not use the indexer tensors (indexer runtime not yet implemented), so they are loaded-but-unused. Marking them TENSOR_NOT_REQUIRED lets layers without an indexer load as nullptr and the model runs as full MLA attention.

DeepSeek-V3.2 (uniform indexer on all layers) is unaffected.