[BUG] Learning rate not overrided when set `--override-opt_param-scheduler`

**Describe the bug**
When setting `override-opt_param-scheduler` (but still load optimizer and load rng) and setting new learning rate scheduler (including max lr, min lr, decay style, etc.), the learning rate still persists its original scheduler.

A related issue could be https://github.com/NVIDIA/Megatron-LM/issues/963

**To Reproduce**
1. Set max_lr as 6e-4 and constant learning rate.
2. Training some steps and save checkpoint.
3. Load the checkpoint (including optimizer params) and override the scheduler (e.g. cosine 6e-4 to 6e-5).
4. Then the bug shows: the learning rate is still constantly 6e-4.

**Expected behavior**
The learning rate scheduler should be overrided.

**Environment (please complete the following information):**
- Megatron-LM commit ID [9bcd4175bec]

**Proposed fix**
The following code (https://github.com/NVIDIA/Megatron-LM/blob/main/megatron/core/optimizer_param_scheduler.py#L121) leads to reloading the optimizer's learning rate when setting `override-opt_param-scheduler`
```python
def get_lr(self, param_group: dict) -> float:
"""Learning rate decay functions from:
https://openreview.net/pdf?id=BJYwwY9ll pg. 4

Args:
param_group (dict): parameter group from the optimizer.
"""

max_lr = param_group.get('max_lr', self.max_lr)
min_lr = param_group.get('min_lr', self.min_lr)
```

A possible solution could be
```python
def get_lr(self, param_group: dict) -> float:
"""Learning rate decay functions from:
https://openreview.net/pdf?id=BJYwwY9ll pg. 4

Args:
param_group (dict): parameter group from the optimizer.
"""

max_lr = self.max_lr
min_lr = self.min_lr
```