Do not load constant_segment if only the placeholder exists (#5229)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5229

1. If constant segment offsets only contains one value, it is the placeholder value for non-const tensors. This means the constant segment is empty, and does not need to be loaded. (T201034228)

2. Clean up before returning error in `prepare_input_tensors`. (T201103889)

Reviewed By: dbort

Differential Revision: D62455987

fbshipit-source-id: ecd3ff792e4501b00c778242e89bdb97a6211e64