Как убрать отступы по вертикали между лейблами PyQT

Как вариант:

void QBoxLayout::addStretch(int stretch = 0)

Добавляет растягиваемое пространство (QSpacerItem) с нулевым минимальным размером и коэффициентом растяжения stretch в конец этого макета блока.

import sys
from PyQt5.QtWidgets import QApplication, QMainWindow, QTabWidget, \
    QWidget, QVBoxLayout, QHBoxLayout, QLabel, QPushButton, \
    QRadioButton, QLineEdit, QCheckBox, QDockWidget, QSizePolicy
from PyQt5.QtCore import QTimer, QDateTime, Qt


class MainApplication(QMainWindow):
    def __init__(self):
        super().__init__()
        self.setWindowTitle("Графический интерфейс")
        self.setGeometry(0, 0, 800, 600)

        self.create_menu_bar()
        self.create_side_panel()

        self.central_widget = QTabWidget()
        self.setCentralWidget(self.central_widget)
        self.central_widget.setStyleSheet("background-color: rgb(200, 222, 222);")

        self.create_smart_house_tab()
        self.create_tab("Вкладка 2")
        self.create_tab("Вкладка 3")
        self.create_tab("Вкладка 4")
        self.create_tab("Вкладка 5")

    def create_menu_bar(self):
        menu_layout = QVBoxLayout()

        self.date_label = QLabel(self)
        menu_layout.addWidget(self.date_label, alignment=Qt.AlignHCenter)

        menu_widget = QWidget(self)
        menu_widget.setLayout(menu_layout)
        self.setMenuWidget(menu_widget)

        self.update_date_time()

    def create_side_panel(self):
        side_panel = QDockWidget("Side Panel", self)
        side_panel.setFixedWidth(120)
        side_panel.setFeatures(QDockWidget.NoDockWidgetFeatures)
        side_panel.setWidget(self.create_side_panel_content())

        self.addDockWidget(Qt.LeftDockWidgetArea, side_panel)

    def create_side_panel_content(self):
        side_panel_content = QWidget(self)
        side_layout = QVBoxLayout(side_panel_content)

        for i in range(1, 6):
            button = QPushButton(f"Вкладка {i}", self)
            button.setSizePolicy(QSizePolicy.Fixed, QSizePolicy.Fixed)
            button.clicked.connect(lambda _, index=i: self.show_tab(index))
            # Добавляем стиль для скругленных углов, голубого цвета и  размера
            button.setStyleSheet(
                "border-radius: 5px; padding: 5px; margin: 0px; "
                "background-color: #00BFFF; color: white; font-size: 18px;")
            side_layout.addWidget(button, alignment=Qt.AlignHCenter)
        return side_panel_content

    def create_smart_house_tab(self):
        tab = QWidget()
        self.central_widget.addTab(tab, "Smart House")
        
        smart_house_label = QLabel("Smart House", self, 
            alignment=Qt.AlignTop | Qt.AlignHCenter)                          # +++
        smart_house_label.setStyleSheet(
            "font-size: 24px; background-color: #775;")

        switch_label = QLabel("Переключатели", self, 
            alignment=Qt.AlignTop|Qt.AlignHCenter)                            # +++
        relay1_label = QLabel("Реле1", self)
        self.central_widget.setStyleSheet("background-color: #9bc;")
        
        relay1_on_radio = QRadioButton("Вкл.", self)
        relay1_off_radio = QRadioButton("Выкл.", self)

        relay2_label = QLabel("Реле2", self)
        relay2_on_radio = QRadioButton("Вкл.", self)
        relay2_off_radio = QRadioButton("Выкл.", self)

        relay3_label = QLabel("Реле3", self)
        relay3_on_radio = QRadioButton("Вкл.", self)
        relay3_off_radio = QRadioButton("Выкл.", self)

        relay1_off_radio.setChecked(True)
        relay2_off_radio.setChecked(True)
        relay3_off_radio.setChecked(True)

        layout = QVBoxLayout(tab)
#        layout.addWidget(smart_house_label, alignment=Qt.AlignTop|Qt.AlignHCenter)
        layout.addWidget(smart_house_label)                                        # +++

        switch_layout = QVBoxLayout()
#        switch_layout.addWidget(switch_label, alignment=Qt.AlignTop|Qt.AlignHCenter)
        switch_layout.addWidget(switch_label)                                      # +++
        
        # Размещаем радиокнопки для Реле1
        relay1_layout = QHBoxLayout()
        relay1_layout.addStretch(1)                                                # +++
        relay1_layout.addWidget(relay1_label)
        relay1_layout.addWidget(relay1_on_radio)
        relay1_layout.addWidget(relay1_off_radio)
        switch_layout.addLayout(relay1_layout)
        relay1_layout.addStretch(1)                                                # +++
        # Размещаем радиокнопки для Реле2
        relay2_layout = QHBoxLayout()
        relay2_layout.addStretch(1)                                                # +++
        relay2_layout.addWidget(relay2_label)
        relay2_layout.addWidget(relay2_on_radio)
        relay2_layout.addWidget(relay2_off_radio)
        relay2_layout.addStretch(1)                                                # +++
        switch_layout.addLayout(relay2_layout)

        # Размещаем радиокнопки для Реле3
        relay3_layout = QHBoxLayout()
        relay3_layout.addStretch(1)                                                # +++
        relay3_layout.addWidget(relay3_label)
        relay3_layout.addWidget(relay3_on_radio)
        relay3_layout.addWidget(relay3_off_radio)
        relay3_layout.addStretch(1)                                                # +++
        switch_layout.addLayout(relay3_layout)

        layout.addLayout(switch_layout)
# +++
        switch_layout.addStretch(1)                                                # +++

        sensors_label = QLabel("Датчики", self)
        sensor1_label = QLabel("Датчик1", self)
        value_label = QLabel("12345", self)

        sensors_layout = QVBoxLayout()
        sensors_layout.addWidget(sensors_label, alignment=Qt.AlignTop | Qt.AlignHCenter)
        sensor_layout = QHBoxLayout()
        sensor_layout.addWidget(sensor1_label, alignment=Qt.AlignTop | Qt.AlignHCenter)
        sensor_layout.addWidget(value_label, alignment=Qt.AlignTop | Qt.AlignHCenter)
        sensors_layout.addLayout(sensor_layout)
        layout.addLayout(sensors_layout)

    def create_tab(self, tab_name):
        tab = QWidget()
        self.central_widget.addTab(tab, tab_name)

        label = QLabel("Текст:", self)
        entry = QLineEdit(self)
        button = QPushButton("Нажми меня", self)
        checkbox = QCheckBox("Чекбокс", self)

        layout = QVBoxLayout(tab)
        layout.addWidget(label, alignment=Qt.AlignTop | Qt.AlignHCenter)
        layout.addWidget(entry, alignment=Qt.AlignTop | Qt.AlignHCenter)
        layout.addWidget(button, alignment=Qt.AlignTop | Qt.AlignHCenter)
        layout.addWidget(checkbox, alignment=Qt.AlignTop | Qt.AlignHCenter)

    def show_tab(self, index):
        self.central_widget.setCurrentIndex(index - 1)

    def update_date_time(self):
        current_time = QDateTime.currentDateTime().toString("yyyy-MM-dd HH:mm:ss")
        self.date_label.setText(f"Текущее время: {current_time}")
        self.date_label.setStyleSheet("font-size: 16px;")  # Увеличиваем размер шрифта
        QTimer.singleShot(1000, self.update_date_time)


if __name__ == "__main__":
    app = QApplication(sys.argv)
    main_app = MainApplication()
    main_app.show()  
    sys.exit(app.exec_())

P.S. Про расположение "Датчиков" вы ничего не писали.