Fix GRU functional API model with return_state=True throwing KeyError #102791
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Fixes tensorflow#102598 When using GRU (or other RNN layers) with return_state=True in a Functional API model, various KeyError exceptions were thrown during model compilation when only one output was used. The issue occurred when a model had a single output from a multi-output layer (e.g., only sequences from GRU with return_state=True), but the loss/metrics were provided as a nested structure (list or tuple). This fix adds handling in map_to_output_names() to unwrap single-element nested structures when the model has a single output, matching the expected structure for loss and metric routing.
mihaimaruseac
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Oct 24, 2025
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Fixes #102598
When using GRU (or other RNN layers) with return_state=True in a Functional API model, various KeyError exceptions were thrown during model compilation when only one output was used.
The issue occurred when a model had a single output from a multi-output layer (e.g., only sequences from GRU with return_state=True), but the loss/metrics were provided as a nested structure (list or tuple).
This fix adds handling in map_to_output_names() to unwrap single-element nested structures when the model has a single output, matching the expected structure for loss and metric routing.