fix: All null column rendering

[Ahzyuan]

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Type of changes

• 🐞 Bug fix
• 🌟 New feature
• 🧪 Tests
• 🏗 Refactoring
• ⚡ Performance improvement
• 📖 Documentation / docstrings
• ↩️ Revert
• 🧩 Other

Description

Purpose

The purpose of this PR is to prevent an error from occurring during the rendering of the tabular report in the event that one or more columns have all null values.

Changes:

• torchmeter/display.py: use column type to identify a all null column, so as to treat it with a different method.
• tests/test_display.py: add an all-null column in fixture example_df so as to simulate the case talking here. By the way, modify all cases related to the structure/content of example_df.

[github-actions]

Coverage Details on macos under Python 3.8

File	Stmts	Miss	Cover	Missing
_stat_numeric.py	113	0	100%
config.py	280	10	96%	330, 367, 509, 515, 579–584
core.py	370	13	96%	423–424, 429, 467–468, 529, 671, 736, 799, 863, 1827, 1833, 1838
display.py	437	6	98%	83, 437–438, 440, 803, 905
engine.py	106	0	100%
statistic.py	443	15	96%	54, 60, 66, 72, 77, 207, 792, 850–855, 868–869
unit.py	41	0	100%
utils.py	127	1	99%	71
TOTAL	1917	45	97%

Summary (on macos under Python 3.8)

Tests	Skipped	Failures	Errors	Time
806	2 💤	0 ❌	0 🔥	7.917s ⏱️

[github-actions]

Coverage Details on ubuntu under Python 3.8

File	Stmts	Miss	Cover	Missing
_stat_numeric.py	113	0	100%
config.py	280	10	96%	330, 367, 509, 515, 579–584
core.py	370	13	96%	423–424, 429, 467–468, 529, 671, 736, 799, 863, 1827, 1833, 1838
display.py	437	6	98%	83, 437–438, 440, 803, 905
engine.py	106	0	100%
statistic.py	443	15	96%	54, 60, 66, 72, 77, 207, 792, 850–855, 868–869
unit.py	41	0	100%
utils.py	127	1	99%	71
TOTAL	1917	45	97%

Summary (on ubuntu under Python 3.8)

Tests	Skipped	Failures	Errors	Time
806	2 💤	0 ❌	0 🔥	11.632s ⏱️

[github-actions]

Coverage Details on windows under Python 3.8

File	Stmts	Miss	Cover	Missing
_stat_numeric.py	113	0	100%
config.py	280	10	96%	330, 367, 509, 515, 579–584
core.py	370	13	96%	423–424, 429, 467–468, 529, 671, 736, 799, 863, 1827, 1833, 1838
display.py	437	6	98%	83, 437–438, 440, 803, 905
engine.py	106	0	100%
statistic.py	443	15	96%	54, 60, 66, 72, 77, 207, 792, 850–855, 868–869
unit.py	41	0	100%
utils.py	127	1	99%	71
TOTAL	1917	45	97%

Summary (on windows under Python 3.8)

Tests	Skipped	Failures	Errors	Time
806	2 💤	0 ❌	0 🔥	13.187s ⏱️