Minor fix for the NamedTensorInfoMap type.

[jinjingforever]

Without this, the build would fail in Angular apps that use typescript 4+ which has more strict check.

The failed code:

export interface FusedConv2DInputs extends NamedTensorInfoMap {
  x: TensorInfo;
  filter: TensorInfo;
  bias?: TensorInfo;  // <--- Error: Property 'bias' of type 'TensorInfo | undefined' is not assignable to string index type 'TensorInfo'.
  preluActivationWeights?: TensorInfo;  // <--- Same error
}

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