Fix ffn_down quantization mix for MoE models (#4927)

Fix ffn_down quantization mix for MoE models (#4927)

* Fix ffn_down quantization mix for MoE models

In #4872 I did not consider the part where every third
tensor is quantized with more bits. Fir MoE this leads to tensors
of the same layer being quantized with different number of bits,
which is not considered as a possibility in the inference implementation
(it is assumed all experts use the same quantization).

* Fix the fix

* Review suggestion

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Co-authored-by: Iwan Kawrakow <redacted>