Fix GGUF to Work Better with modules_to_not_convert / keep_in_fp32_modules

[dg845]

What does this PR do?

This PR contains several fixes so the GGUF loading and inference work better with module_to_not_convert and _keep_in_fp32_modules.

Changelist

1. src/diffusers/quantizers/gguf/utils.py
   1. _replace_with_gguf_linear: adds a check to see if any of the current module's named_children are in modules_to_not_convert, and if so, skip it. This allows us skip containers, rather than just leaf-level nn.Linear submodules as in the current code. For example, TimestepEmbedding modules are commonly added to _keep_in_fp32_modules (e.g. time_embedder in WanTransformer3DModel's WanTimeTextImageEmbedding condition embedder), but since they themselves contain leaf nn.Linear submodules such as linear_1, the current code will only check against leaf modules such as linear_1, and conclude incorrectly that they should be converted.
   2. _fused_mul_mat_gguf: in the UNQUANTIZED_TYPES case, also cast the dequantized weight to the activation x's dtype before performing the matrix multiplication, which should prevent dtype errors for BF16 weights.
2. src/diffusers/quantizers/gguf/gguf_quantizer.py
   1. GGUFQuantizer.create_quantized_param: handles modules_to_not_convert by dequantizing them, so that they end up in their original unquantized form. This is intended to handle the case where a module in self.modules_to_not_convert (or one of its children) is in the GGUF file. Since it is in the file, it will be converted to a GGUFParameter, but we don't want it to be quantized, so we convert it back here.

Inspired by GGUF debugging in #13551, in particular #13551 (comment).

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