Update utils.py

[Vanchrn]

Description

In /contrib/strategy/signal_strategy.py, it comments "get the number of trading step finished, trade_step can be [0, 1, 2, ..., trade_len - 1]" If set finished == self.trade_step >= self.trade_len it will raise IndexError: index X is out of bounds for axis 0 with size X. I think change self.trade_len = _end_index - _start_index + 1 to self.trade_len = _end_index - _start_index maybe better, but it may result in other errors.

Motivation and Context

How Has This Been Tested?

• [x] Pass the test by running: pytest qlib/tests/test_all_pipeline.py under upper directory of qlib.
• [ ] If you are adding a new feature, test on your own test scripts.

Screenshots of Test Results (if appropriate):

1. Pipeline test:
2. Your own tests:

Types of changes

• [x] Fix bugs
• [ ] Add new feature
• [ ] Update documentation