Rename task -> input in dataset.py (#467)

*Issue #, if available:*

*Description of changes:*

Rename all variables and methods that refer to "task" in `dataset.py` to
use `input` instead:
- `PreparedTask` → `PreparedInput`
- `self.tasks` → `self.inputs`
- `prepare_tasks` → `prepare_inputs`
- `validate_and_prepare_single_dict_task` →
`validate_and_prepare_single_dict_input`
- All `task_` prefixed local variables renamed (e.g., `task_target` →
`target`, `task_context` → `context`, `task_past_tensor` →
`past_tensor`, etc.)


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Author: shchur