Protect for missing slack_sdk import

[hanlint]

What does this PR do?

Fixes #2030 . slack_sdk is not in the base install, so the HealthChecker needs to check the import.

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[mvpatel2000]

Can we gate pynvml the same way?

[dakinggg]

manual testing evidence

In [1]: import slack_sdk
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
Cell In[1], line 1
----> 1 import slack_sdk

ModuleNotFoundError: No module named 'slack_sdk'

In [2]: from composer.callbacks import HealthChecker

In [3]: hc = HealthChecker(slack_webhook_url='test')
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
File ~/github/composer/composer/callbacks/health_checker.py:75, in HealthChecker.__init__(self, threshold, sample_freq, window_size, wait, slack_webhook_url, test_mode)
     74 try:
---> 75     import slack_sdk as slack_sdk
     76 except ImportError as e:

ModuleNotFoundError: No module named 'slack_sdk'

The above exception was the direct cause of the following exception:

MissingConditionalImportError             Traceback (most recent call last)
Cell In[3], line 1
----> 1 hc = HealthChecker(slack_webhook_url='test')

File ~/github/composer/composer/callbacks/health_checker.py:77, in HealthChecker.__init__(self, threshold, sample_freq, window_size, wait, slack_webhook_url, test_mode)
     75         import slack_sdk as slack_sdk
     76     except ImportError as e:
---> 77         raise MissingConditionalImportError('health_checker', 'slack_sdk') from e
     79 self.last_sample = 0
     80 self.last_check = 0

MissingConditionalImportError: Composer was installed without health_checker support. To use health_checker related packages, with Composer, run `pip install 'mosaicml[health_checker]'` if using pip or `conda install -c conda-forge slack_sdk` if using Anaconda.

[dakinggg]

New round of manual checks

without pynvml or slack_sdk

In [1]: from composer.callbacks import SpeedMonitor, HealthChecker

In [2]: hc = HealthChecker()
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
File /workdisk/danielking/github/composer/composer/callbacks/health_checker.py:146, in HealthChecker._is_available()
    145 try:
--> 146     import pynvml
    147     pynvml.nvmlInit()  # type: ignore

ModuleNotFoundError: No module named 'pynvml'

During handling of the above exception, another exception occurred:

MissingConditionalImportError             Traceback (most recent call last)
Cell In[2], line 1
----> 1 hc = HealthChecker()

File /workdisk/danielking/github/composer/composer/callbacks/health_checker.py:77, in HealthChecker.__init__(self, threshold, sample_freq, window_size, wait, slack_webhook_url, test_mode)
     74 self.last_check = 0
     76 self.metrics = []
---> 77 if self._is_available():
     78     self.metrics.append(GPUUtilization(threshold))

File /workdisk/danielking/github/composer/composer/callbacks/health_checker.py:150, in HealthChecker._is_available()
    148     return True
    149 except ImportError:
--> 150     raise MissingConditionalImportError('health_checker', 'pynvml', None)
    151 except pynvml.NVMLError_LibraryNotFound:  # type: ignore
    152     logging.warning('NVML not found, disabling GPU health checking')

MissingConditionalImportError: Composer was installed without health_checker support. To use health_checker related packages, with Composer, run `pip install 'mosaicml[health_checker]'` if using pip or `pip install pynvml` if using Anaconda.

with pynvml, without slack_sdk

In [1]: from composer.callbacks import SpeedMonitor, HealthChecker

In [2]: hc = HealthChecker()

In [3]: hc._is_available()
Out[3]: True

In [4]: hc = HealthChecker(slack_webhook_url='test')
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
File /workdisk/danielking/github/composer/composer/callbacks/health_checker.py:68, in HealthChecker.__init__(self, threshold, sample_freq, window_size, wait, slack_webhook_url, test_mode)
     67 try:
---> 68     import slack_sdk
     69     del slack_sdk

ModuleNotFoundError: No module named 'slack_sdk'

The above exception was the direct cause of the following exception:

MissingConditionalImportError             Traceback (most recent call last)
Cell In[4], line 1
----> 1 hc = HealthChecker(slack_webhook_url='test')

File /workdisk/danielking/github/composer/composer/callbacks/health_checker.py:71, in HealthChecker.__init__(self, threshold, sample_freq, window_size, wait, slack_webhook_url, test_mode)
     69         del slack_sdk
     70     except ImportError as e:
---> 71         raise MissingConditionalImportError('health_checker', 'slack_sdk', None) from e
     73 self.last_sample = 0
     74 self.last_check = 0

MissingConditionalImportError: Composer was installed without health_checker support. To use health_checker related packages, with Composer, run `pip install 'mosaicml[health_checker]'` if using pip or `pip install slack_sdk` if using Anaconda.

with pynvml and slack_sdk

In [1]: from composer.callbacks import SpeedMonitor, HealthChecker

In [2]: hc = HealthChecker(slack_webhook_url='test')

In [3]: hc._is_available()
Out[3]: True

[dakinggg]

LGTM