Fix: Fix cal_bandgap for parallelism of k-points and band.

[sunliang98]

Reminder

• Have you linked an issue with this pull request?
• Have you added adequate unit tests and/or case tests for your pull request?
• Have you noticed possible changes of behavior below or in the linked issue?
• Have you explained the changes of codes in core modules of ESolver, HSolver, ElecState, Hamilt, Operator or Psi? (ignore if not applicable)

Linked Issue

Fix #6601

Unit Tests and/or Case Tests for my changes

• A unit test is added for each new feature or bug fix.

What's changed?

This PR fixes inconsistencies in parallel computation results. Below are two examples demonstrating the improvements:

1. K-points Parallelism (01_PW/004_PW_UPF201_Si, Si)

Before fix:

kpar	E_bandgap (Ry)
1	0.0298852541
2	0.1033124558
3	0.1762318244

After fix:

kpar	E_bandgap (Ry)
1	0.0298852541
2	0.0298852541
3	0.0298852541

3. Band Parallelism (11_PW_GPU/001_PW_BPCG_GPU, GaAs, run with CPU)

Before fix:

nCPU	E_bandgap (Ry)
1	0.0416524880
4	inf

After fix:

nCPU	E_bandgap (Ry)
1	0.0416524880
4	0.0416513276

Results are now consistent across different parallel configurations

Any changes of core modules? (ignore if not applicable)

• Example: I have added a new virtual function in the esolver base class in order to ...