[Minor] Reduce DataFrame copies

[ourownstory]

Remove all copies of DataFrame except for top-level function call (when user passes df), and few necessary exceptions.

Background: Due to various pieces of code contributed by different contributors, unnecessary copies of the DataFrame were made out of precaution. This is here being addressed.

[github-actions]

Model Benchmark

Benchmark	Metric	main	current	diff
PeytonManning	MAE_val	0.35033	0.35033	0.0%	✅
PeytonManning	RMSE_val	0.50095	0.50095	0.0%	✅
PeytonManning	Loss_val	0.01775	0.01775	0.0%	✅
PeytonManning	MAE	0.34667	0.34667	0.0%	✅
PeytonManning	RMSE	0.49357	0.49357	0.0%	✅
PeytonManning	Loss	0.01466	0.01466	0.0%	✅
PeytonManning	LR	0.00032	0.00032	0.0%	✅
PeytonManning	time	15.3208	14.77	-3.6%	✅
AirPassengers	MAE_val	30.1315	30.1315	0.0%	✅
AirPassengers	RMSE_val	31.0835	31.0835	0.0%	✅
AirPassengers	Loss_val	0.01242	0.01242	0.0%	✅
AirPassengers	MAE	6.12226	6.12226	0.0%	✅
AirPassengers	RMSE	7.80921	7.80921	0.0%	✅
AirPassengers	Loss	0.00064	0.00064	0.0%	✅
AirPassengers	LR	0.0004	0.0004	0.0%	✅
AirPassengers	time	8.42425	8.34	-1.0%	✅
EnergyPriceDaily	MAE_val	5.40186	5.40186	0.0%	✅
EnergyPriceDaily	RMSE_val	6.70655	6.70655	0.0%	✅
EnergyPriceDaily	Loss_val	0.02519	0.02519	0.0%	✅
EnergyPriceDaily	MAE	5.91309	5.91309	0.0%	✅
EnergyPriceDaily	RMSE	7.94008	7.94008	0.0%	✅
EnergyPriceDaily	Loss	0.02554	0.02554	0.0%	✅
EnergyPriceDaily	LR	0.00029	0.00029	0.0%	✅
EnergyPriceDaily	time	19.4359	19.89	2.34%	✅
YosemiteTemps	MAE_val	0.57336	0.57336	0.0%	✅
YosemiteTemps	RMSE_val	0.84714	0.84714	0.0%	✅
YosemiteTemps	Loss_val	0.00042	0.00042	0.0%	✅
YosemiteTemps	MAE	0.93956	0.93956	0.0%	✅
YosemiteTemps	RMSE	1.65601	1.65601	0.0%	✅
YosemiteTemps	Loss	0.00118	0.00118	0.0%	✅
YosemiteTemps	LR	0.0001	0.0001	0.0%	✅
YosemiteTemps	time	58.8034	58.23	-0.98%	✅

Model training plots

Model Training

PeytonManning

YosemiteTemps

AirPassengers

EnergyPriceDaily